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This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your own answer and add a link to it in here (or wait for me to do it). Ideally, this answer should just be a summary (with short entries while individual other answers would have the details).

Currently actively maintained systems:

• Cygwin. A POSIX layer for Microsoft Windows. Used for Windows UNC paths.
• UWIN since 1.3. Another POSIX layer for Windows. Used at least for //host/file network file sharing paths.
• ^{@OlivierDulac} IBM z/OS as mentioned in the POSIX bug tracker, z/OS resolves //pathname requests to MVS datasets, not to network files. Example.

Defunct systems

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

  According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.

• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)

• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.

• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

Applications that treat //foo/bar specially for paths

• ^@Prem Perforce where //depot/A/B/C/D refers to a path in a depot.
• ^@WChargin Blender. In its configuration you use a // prefix for relative paths (to the blend associated with the data-block).

This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your own answer and add a link to it in here (or wait for me to do it). Ideally, this answer should just be a summary (with short entries while individual other answers would have the details).

Currently actively maintained systems:

• Cygwin. A POSIX layer for Microsoft Windows. Used for Windows UNC paths.
• UWIN since 1.3. Another POSIX layer for Windows. Used at least for //host/file network file sharing paths.
• ^{@OlivierDulac} IBM z/OS as mentioned in the POSIX bug tracker, z/OS resolves //pathname requests to MVS datasets, not to network files. Example.

Defunct systems

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

  According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.

• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)

• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.

• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

Applications that treat //foo/bar specially for paths

• ^@Prem Perforce where //depot/A/B/C/D refers to a path in a depot.
• ^@WChargin Blender. In its configuration you use a // prefix for relative paths (to the blend associated with the data-block).
• The Bazel build system uses a // prefix for labels of targets within the Bazel build graph.

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.
• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)
• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.
• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

  According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.

• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)

• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.

• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your own answer and add a link to it in here (or wait for me to do it). Ideally, this answer should just be a summary (with short entries while individual other answers would have the details).

Currently actively maintained systems:

• Cygwin. A POSIX layer for Microsoft Windows. Used for Windows UNC paths.
• UWIN since 1.3. Another POSIX layer for Windows. Used at least for //host/file network file sharing paths.
• ^{@OlivierDulac} IBM z/OS as mentioned in the POSIX bug tracker, z/OS resolves //pathname requests to MVS datasets, not to network files. Example.

Defunct systems

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.
• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)
• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.
• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

Applications that treat //foo/bar specially for paths

• ^@Prem Perforce where //depot/A/B/C/D refers to a path in a depot.
• ^@WChargin Blender. In its configuration you use a // prefix for relative paths (to the blend associated with the data-block).

This is a compilation and index of the answers given so far. This post is community wiki, it can be edited by anybody with 100+ reputation and nobody gets reputation from it. Feel free to post your own answer and add a link to it in here (or wait for me to do it). Ideally, this answer should just be a summary (with short entries while individual other answers would have the details).

Currently actively maintained systems:

• Cygwin. A POSIX layer for Microsoft Windows. Used for Windows UNC paths.
• UWIN since 1.3. Another POSIX layer for Windows. Used at least for //host/file network file sharing paths.
• ^{@OlivierDulac} IBM z/OS as mentioned in the POSIX bug tracker, z/OS resolves //pathname requests to MVS datasets, not to network files. Example.

Defunct systems

• ^@BinaryZebra Apollo Domain/OS (confirmed). Also mentioned at Official Description UNC (Universal Naming Convention) as the possible origin of //host/path notations (see also, page 2-15).

According to Donn Terry, it was HP (which acquired Apollo Computers) that pushed for inclusion of that provision in the POSIX spec for Domain/OS.

• ^@jillagre Tektronix Utek (corroborated), where //host/path is a path on a distributed file system.
• ^@gilles QNX 4 with the FLEET distributed processing system, where //123/path is a /path on node 123. (Mentioned in the QNX 6 documentation.)
• ^@roaima AT&T SysV Release 3 (unverified). //host/path in (discontinued in SVR4) RFS Remote File Sharing system.
• ^@Scott SEL/Gould UTX-32 (unverified). Used for //host/path.

Applications that treat //foo/bar specially for paths

• ^@Prem Perforce where //depot/A/B/C/D refers to a path in a depot.
• ^@WChargin Blender. In its configuration you use a // prefix for relative paths (to the blend associated with the data-block).