I use several open source tools in my day to day development - and they save me a lot of money.
In the absense of donating any time to these projects I decided to donate some cash to a project once per year.
If you were going to donate $100 which project would get your vote?
The JavaScript framework of your choice. Dealing with all of the cross-browser idiosyncrasies is a nightmare and the framework developers save a lot of web developers a lot of time by dealing with those issues.
You should read Jeff's post about donating $5000 to an open source project and if open source projects can actually benefit from donations. Basically, the project coordinator said that the money sat for three months without being touched because they couldn't find an appropriate usage for it.
Anyway, I find the open source projects that affect me the most are the big ones - Linux, Firefox, GNU, SVN, etc. I think the best thing for OSS is spreading the word, increasing adoption, awareness, and participants.
I'd spread out the $100 (if that is your donation budget) and donate to different projects. If you is a company, you might as well add a 0. :)
The smallest project you use that gives you the most benefit probably needs and deserves your money the most.
valgrind is a pretty useful tool for checking profiling, and is well maintained.
Donating to a one-man project rather than something a lot larger like the Mozilla Foundation is probably a better use of $100. It will directly encourage him to work on that product and also give him a good feeling that people out there use his software and are grateful for it.