AudioManager is a must! It's the best volume control app available.

Other honorable mentions: Titanium Backup, Stitcher, Evernote, Dropbox, Netflix, Music (music.google.com, not stock), SMS Backup, CallTrack (logs all incoming and outgoing calls directly into your Google Calendar), and lastly, both Autorun Manager and AutoKiller Memory. The last two help stop annoying stock programs from auto launching on triggers (like plugging in headphones) and from running in the background.