Apps that I use day in day out…
Hahlo – not strictly an iPhone app, but the best mobile Twitter client that I’ve found. It does everything I need of it (apart from maybe twitpic), it’s quick to use and looks pretty. The only downside it that as it lives in Safari you occasionally get unwanted refreshes so you loose you place in the list of tweets.
Facebook – I’d pretty much stopped using Facebook before installing this app, but now I check it once or twice a day. It just makes it so much easier to check for updates and messages.
Linked In – This app is much the same as the Facebook app, but perhaps slightly less well executed. I have to say I use Linked In less, but it’s still a worthwhile app if you have an account.
Tris – I always was a fan of Tetris :)
Evernote – A fantastic tool, if you don’t have an account you should go and try it. I love the way I can take a photo on the phone, it’s then uploaded to Evernote, OCR’d and indexed. It makes recording notes from whiteboards so much easier, just take a photo then search for keywords.