dropbox for syncing stuff from your phone to your pc
Skype for calling people for free/cheap
Wolfram Alpha for never doing any sort of math ever again
Astro for browsing the filesystem effectively
Speed test, to see how slow your non-LTE 4G really is

(Verizon FTW)
Gameboid (GBA emulator), n64oid (n64), GBCoid (GBC) for when you run out of native android games
Pandora, for when it is to awkward to play games, and you run out of MP3s
WinAmp to replace the cruddy stock media player (the HTC one is decent, but eeeehhhh not great)
cant think of anything else ATM