I have 2 top five lists. An official and a Jailbreak so here goes:
Orb Live - Media Streaming app that lets me watch whatever I want on my iPhone, as long as it's on my computer and the PC is turned on, which is 24 hours a day lol. I do find myself using this more on the iPad than iPhone and more so away from home!
Stumble Upon - Portable version of the Stumble Upon website. When I am stuck for something to do there is something quite satisfying about being taken to random websites. You stumble across a few fantastic sites that you never knew existed in a 15-30 minute session. Half my bookmarked sites are made up from stumbled ones.
TuneIn Radio - Digital Radio player with 40,000+ stations! only 59p or I assume 99c? Can't really say any more than that... I love radio and love it on the move!
Twitter (official app) - Doesn't require an explanation, I have just got into twitter recently and have caught the bug big time. Feel free to add me @tiphub
Fone Jacker - Won't mean much to people outside of the UK but my god I have some fun with this app, basically its a comedian over here who does prank phone calls on tele and you get all his common phrases in the app... point your speaker to another phone and have some fun with the caller on the end of the line! You should check this guy out if you can get 4On Demand in the states.
JailBreak Apps (Sorry if this shouldn't be included please delete if not acceptable):
My3G - FaceTime over 3G and much more. I rarely get in from work to see my kids before they go to bed. So a quick call to mum on facetime and a few night nights and blown kisses keeps everyone happy(ish)
WinterBoard - Change the complete look of your iPhone. Make it stand out from everyone else.
UnlimTones - Use you own ring tone from your library. Simple and fast.
MyWi - Turn your iPhone into a wireless router, basically use your 3G connection and share it with a laptop for example. Good for when you need to complete a task that is just not easily done on iOS.
SB Settings - Change simple options like turn wifi on/off turn bluetooth on/off without having to leave an app and go into settings. Should definitely be a standard iOS feature.