The UT Austin chapter of the American Institute of Aeronautics and Astronautics is a student organization for people with an interest in aviation and space.  The AIAA itself is a worldwide organization of over 31,000 members, and is the largest professional organization devoted to the progress of aviation and space science technology.  Here are just a few of the myriad reasons to join:

1. Gives you another reason to go to WRW; keep up to date on what's going on in the department.
2. Meet professors and other students.
3. See cool presentations (aerospace engineers, astronauts, distinguished professors, industry professionals).
4. Go on the field trips (a.k.a. plant tours) - As a member, most trips are paid for.
5. Fun in the sun (volleyball at the park, picnics and campouts at the lake, bowling with faculty, etc.)
6. The occasional broomball game against ASME (just to put those cocky MEs in their place!)
7. Eat pizza at every meeting
8. Looks good on a resume