Unless I use a program all the time, I’d always prefer a free version. And free software often works better than commercial equivalents. Here’s another list, including some I haven’t checked out yet: Resources on Windows opensource and/or freeware - Listible!. (via lifehack.org)
Update: I’ve replaced the Adobe PDF Reader with Foxit Reader, found on this site. Foxit loads almost instantly, even on my work computer: P4 1.6 w/ 256MB. It’s a big improvement over Adobe. Also CCleaner (Crap Cleaner) has just been enhanced dramatically with the addition of strong (1, 3, or 7 pass randomized overwriting) deletion.
I have a few more posts related free software check them out and let me know what I’ve missed!