I'll jump in here:
1337pwn.com xbox live app: let's you keep track of who's on live and who isn't, and call up their profiles to see what they've been playing, etc. Great for those "who's on playing what?" moments where you don't want to go to xbox.com to check, and don't want to power up your 360 if nobody is on either.
Doggcatcher: the best podcast program i've ever seen. Blows away everything else i've tried, blows away iPhone and ITMS podcast service on my old iPhone, just very *VERY* well done overall. Insanely customizable.
G4: great collection of streaming video clips from the G4 TV network, the quality is really, really high, and they're coming out with new content very regularly.
SportsTap: best sports widget/app i've found. Very easy to set up favorite teams, nice customizable refresh rates on the data, widget works well.
VCast Song ID (Verizon): it does basically the same thing as Shazam and SoundHound, and does it free. My hit rate with it has been pretty positive, but (like the other two) techno and jazz give it fits.