SXSW.com
The official site of SXSW, complete with all official showcase schedules, artist bios, song samples, and a fantastic way to discover music from the showcasing artists: the SXSW '09 Music Player. Look for the link on the right side of each page on SXSW.com.

Sched.org
Feeling overwhelmed by the 1800 or so artists showing up for SXSW, playing multiple official and non-official shows in a matter of a few days? One of the coolest sites of last year's conference will be back soon, bringing as much information to one place as possible and allowing you to pick and choose (and share!) your personal SXSW-week schedule. Be sure to check out their BLOG as well!

Do512.com
Do512 lists not only SXSW events, but a plethora of activities in town all year long, divided by type (music/culture/nightlife/etc). So even if you're not able to officially attend SXSW events, you're sure to find something good going on in Austin on this site.
*UPDATE: Check out sxsw2009.do512.com! You can find artists you like and click on "add their shows to my schedule." It is a great way to discover all of the shows the artist is playing (not only their official SXSW showcase appearances).

Donewaiting.com
Donewaiting.com keeps a timely blog of SXSW happenings, including information of private VIP and RSVP-only shows, as well as free daytime parties going on throughout the week of SXSW.