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Your current implementation could be more efficient. Copying the passed in string and chopping it up is probably pretty expensive (I'm not a master of .NET internals though).

public static int Occurences(this string str, string val)
{  
    int occurrences = 0;
    int startingIndex = 0;

    while ((startingIndex = str.IndexOf(val, startingIndex)) >= 0) 
    {
        ++occurrences;
        ++startingIndex;
    }

    return occurrences;
}

As a note, I'm unaware of the consequences of calling IndexOf with a startingIndex >= the length of the parent string. You may need to check for this.