Have used all the above would also recommend
Ping - free iPhone to iPhone texts (some smart phones also use ping so it would apply to these too not sure which ones though)
Eirtext - use your free webtexts through this easy to use.
My bill - tells how many minutes, texts, data, mms you have left.
Coin dozer - a silly but addictive game.
Entertain me (entertainment.ie) gives you cinema times and locations
App box - its just one app but it contains loads of other apps. It opens out just like an iPhone set up but the apps are all listed inside (sorry for my bad explanation) includes things like calculator, address book, converter.... saves you having loads of individual apps that clutter up your actual iPhone screen
Ebay, Amazon, Wordpress, facebook, twitter,skype - depending on your needs.
There are thousands of apps out there you just need to see what kind of thing you are interested in and search. I have had my phone for almost a year and still discovering new things.