I have around 80 apps or so, but my most used are the following:

Things (it's a pricey to do list that syncs with the even more pricey desktop version which I don't have. I like it and use it all the time, but there's cheaper to-do list options out there, for instance:

Awesome Note, which just added to-do list functionality. I love this app, you can sync, backup, or restore notes with google docs, and there are a ton of options in terms of organization. I used Evernote for a while, but I like this much, much better.

I use Ace Budget and Cha Ching to manage finances, Grocery Gadget to set up grocery lists, Cooliris to browse youtube and flickr, eBuddy to IM, Skype to call home, DropBox to easily transfer things between work, home, and my iPhone, Epicurious for recipes, Comparis apps to check grocery sales and compare prices of electronics when I'm in the store, OffMaps to download maps when I'm traveling to avoid roaming charges, Metro to see all the public transportation timetables of pretty much every city, SBB and Wemlin to check local timetables, Öffnungszeiten to see what stores are still open around me when it's later in the day, GoodReader to read and bookmark those huge PDFs, Flickit to upload my photos to Flickr, Couch to 5k and Runkeeper to do my running, Geocaching is pretty self explanatory, iScore is a great baseball scoring program I use, I listen to live baseball games using MLB At Bat, Public Radio to listen to all my favorite NPR stuff, Stanza for eBooks, WeatherPro is a fantastic weather app with near-realtime sattelite and radar feeds, and Words with Friends is my scrabble substitute. There's plenty more, but that's a start.