ttcp is a simple, possibly too simple, speed test utility.
pchar is another one people cite a lot, I've had bad luck with it, personally.
Here's how I'd use ttcp. You need two machines, each with ttcp (http://playground.sun.com/pub/tcp-impl/ttcp/ttcp.c) compiled on them.
HostA % ./ttcp -r -s -p 9401
...
HostB % ./ttcp -s -p 9401 < /boot/vmlinuz
Once you've figured out how to get it to run, try different length files to see how speed varies. Use UDP (-u flag on both reader and sender command line) for even more fun!