One of the first Critical Mass rides in the U.S., Austin's monthly human-powered celebration of all things biking is now in its' 12th year, bigger and better than ever. The mass currently meets at 6:00 pm on the last Friday of the month on the West Mall near 23rd and Guadalupe. In recent years, Critical Mass has grown in size and scope to include hundreds of cyclists of all kinds, from commuter cyclists, to racers, mountain bikers, fixed gear free-stylers, freak bike builders, BMX bandits, and everyone in-between. The rides are leaderless and directionless by design, so show up to be a part of this organized coincidence.
http://cycling.frenzied.us/ has lots of pictures of the Austin Critical Mass rides from 2008 to 2012.
Currently (as of 2015 or so) ... Austin doesn't really have a Critical Mass ride going on. San Antonio and Houston both have rides going on and aren't too far away, however.