Astro File Explorer - Probably the most popular zip/explorer app out there
Vignette - Excellent camera app chock full of effects
Picsay Pro - For photoshopping on the go (better than the actual mobile Photoshop app)
vPlayer - Plays every video I've tried and plays it well. Tried a ton of video players but this is by far the most functional.
Handcent - SMS app that does everything I need it to. Haven't tried GO SMS or Chomp, but haven't had the need to.
Email Notifier - Popup feature for Gmail and K9.
JuiceDefender - Must-have battery saver
Adfree - Must-have app that elminates most ads found in apps and the web. Only ads it couldn't block were the ones on...
Wordfeud - Extremely addictive game in the vein of Scrabble or Words with Friends. Play as time permits. Can have a bunch of sessions open when other players are unavailable.
Tune In - App that lets you stream radio stations all over the world.