Script Summary: When editing your Comment Signature, this userscript shows how many more characters you can type.
When editing your Comment Signature on the Edit Profile page, this script adds a counter showing how many characters you've typed, and how many are allowed.
Update 4/11/2009:Many updates made to the script. Among other things, the counter takes non-ASCII characters into account. (deviantART translates international characters like "ಠ" into codes like "ಠ", so an international character actually counts as four or five characters.) It also removes deviantART's (poorly-done) signature counter.