I realized that tomorrow is probably payday for many of you, so I thought I would remind you that before you start spending that money, remember that the day of your next paycheck falls on Coding Horror's Support Your Favorite Small Software Vendor Day. Last year, I donated to the Enlightenment Window Manager which I use every day. Even thought you aren't limited to just open source projects to donate to (a prime example from the original post's comments is WinZip), I thought I would list off a few projects that I use every day that I will probably be donating to:

Mozilla Foundation

While I've been straying away from Firefox for everyday use, I still use it heavily in web development. I also use Thunderbird as my mail client on all of my systems, and my audio application of choice, Songbird, is based of the XUL API that came from Firefox. While the method of donation is buying things from The Mozilla Store, I will probably end up donating whatever I buy to someone who needs it. This way, I'm helping Mozilla and giving away free stuff!

Songbird

I love Songbird. I never really much cared for the bloat that was iTunes, plus, I just recently got an OS laying around that could run iTunes. However, Songbird's support for podcasts is awesome (even though there's not a single mention of the actual word "podcast"), and building on the extension capabilities of Firefox means the sky is the limit. Their easiest method of donating is, once again, a store so donate whatever you get from their as well. The least you can do is give a shirt to a kid and have him walk around as a travelling billboard.

Vim

I was sold on Vim about a year ago now. I use it as my main development platform. For ~$15 USD, you can become a registered Vim.org user, and for a little more, vote on some of the features you'd like to see implemented.

Free software comes at the cost of someone's time. Just make sure you find a way to say "Thank you" to those humble (if not semi-socialist) developers.