The iTunes Music Store is a year old and celebrating by giving away free singles.
ITMS is also featuring, as part of the new iTunes 4.5, iMix, whereby Store patrons can post their iTunes playlists in a special section of the Store. It's a bit like Amazon reader lists, but with one-click simplicity (since you've already created the list). Obviously, it's a gimmick to further promote sales (only playlist tracks that are actually available through the Store get posted, others are are automatically dropped), not to mention a attractive nuisance for egomaniacal music fans who want to impose their taste on the world, but it's clever nonetheless. Since I fit the latter description, I've already posted a very provisional list I've been working on: Perfect Songs.
One last ITMS note: Derek Powazek has a recent post on the joys of iTunes randomization and notes the newly-added Quick Links in iTunes 4.5, including info on hacking the default behavior of those links.
Update (5/2/04): I've also posted the Four Horsemen mix (or, rather, about half of it) as an iTunes iMix. The other half will have to wait until Apple expands their library.