Here are a few good Austin Viewpoints that were provided to me from KevinFromTexas. Thanks again Kevin!
Butler Park in South Austin
There are some interesting angles on downtown there. There's also a 35 foot tall observation hill. The park is right across the river from downtown.
http://www.ci.austin.tx.us/parks/butler.htm
TMI Castle/Dupont Castle
This was a mansion that was built in the 20s. It sits on a bluff west of downtown. There had been a condo project planned near it, but it was never finished. The foundation for that project was built though, and is still there. It provides a really good place to view the skyline from. You have to enter through a small gravel driveway and through the gate (which is always open).
http://www.dupontcastle.com/castles/tmi.htm
Former Whole Foods garage on Lamar
There are also various parking garages around downtown. The former Whole Foods garage on Lamar Boulevard just west of downtown is a really good place to take pictures. Parking is free, and the security guard is nice about letting you go up for photos. It's located at 603 North Lamar Boulevard.