I recently noticed that the page I was linking to for fonts for my PHP & GD powered captcha script had been taken down, so I went looking for another solution for my users. What I found was even better than another place to get fonts.
GD FontMaker is a script by Howard Yeend that converts PNG images into GD font format. Since it's GPL software, I was able to modify it to make it a little more powerful and easier to use. Download my version here.
The great thing about this script is that it enables you to generate your own unique fonts to use in your captchas. Of course, uniqueness is no guarantee that OCR won't be able to crack your captcha, but it's certainly better than using the same old fonts as everyone else.
The first font I created with this script was apparently a little too easy to crack. I replaced some of the fonts I'd been using before because I periodically get complaints from people who've tried multiple times and can't solve them. Over the next few days, the amount of comment spam in my blogs increased noticeably. It wasn't an explosion, but it was annoying.
So I whipped up a few more fonts by applying filters to the first one (using the GIMP). That seems to have solved the problem.