Check Off is a free little tool that sits discreetly in your menu bar and pops open with a single click to display a basic to do list. I wasn't a big fan of Tiger's native to do list so I started using this a few months ago and it's been a pretty handy little app. Though it looks like Apple finally got its act together by incorporating a to do list into Mail.app, but I don't think I'll give up Check Off entirely since its bare bones functionality has come in rather handy sometimes.
Check Off is mainly for jotting down quick notes on tasks you need to complete so don't expect it to do a lot of heavy lifting. If you just need something to keep you on track or are looking for another way to send a to do list to your iPod, though, then Check Off is just the thing. It's also somewhat tweakable -- you can drag and drop to sort folders, color-code your entries, and attach notes to each to do, if you're so inclined. Version 3.7 has just been released with Leopard compatibility and a few bug fixes.