I have so many apps installed on my phone but my top 5 I use the most are:
IM+ ($9.99)...a solid im app that works with lots of services and uses push so I still get im's when the app is closed. GameCenter (Free) ...newest app on my iphone and really cool. not only can i control my gamefly game q but it taps into a massive database of titles to immediately spit out everything from release dates to platform availability to screenshots to reviews. essential tool for any game fan. Yelp (Free)...better than the site imo, easy to find places around you and read real reviews, makes it easy to bookmark places to try and they have a cool augmented reality function that works on 3GS phone only. PS Mobile (Free)...photoshop has come to the iphone. I can't wait to see future versions. Hold Em ($4.99)...one of the first app for the iphone and still fun to play. nothing like a game of texas hold-em to pass the time.
1)Sportacular- My #1 used sports app, love the push updates for my favorite teams. I'm using this app all the time constantly.
2)Solitaire- Cubes Lite used to be my addiction, now its good old Solitaire.
3)Amazon Kindle- I never read "Real" books anymore. My trick is, I go to book stores, see what books I like, take a picture of it, and then see if I can find it on Amazon for the Kindle.
4)PocketGamer- Love their reviews, previews, and videos of iPhone games
5)300 Bowl- Nothing like a quick game of bowling while on the NYC subways, lol.
I have over 200 apps, love them all, but seem to use these the most.