Below I list all builds that are interesting to me on a single page, so I don't have to look through multiple pages on BetaWiki every time I want to see what's interesting about each build.
3683 and 3706 are the two oldest builds available. They were the only ones with WMP 8.
3713 "is currently the only available pre-reset build to contain both the Media Center and Tablet PC components" (source), but Media Center crashes. WMP is updated to 9.
3718 is the last M3 build.
4001 is the first M4 build, but still uses the M3 wallpaper.
4002 changed to the M4 wallpaper.
4008 is the last build before the boot screen is changed.
4011 is the last M4 build and has a changed boot screen.
4015 is the first M5 build and changed to the M5 wallpaper.
4029 (Lab06_n) is the first M6 build and changed to the M6 wallpaper.
4031 (main) is the last M5 build.