Not sure about hidden gems. But I'll just list the Apps I've got:
Facebook- self explanatory
iDoodle Lite- Fun drawing game
Google Earth- self explanatory
Translator- self explanatory
Cricinfo Mobicast- self explanatory
Virtual Pool- Awesome pool game
PAC-Man- self explanatory
Drinks Free- Quality for a party, just a massive database of various alcoholic drinks and how to make them
PocketGuitar- a guitar in your pocket, quite fun to mess about on
IM+ Lite- Basically MSN in iPhone form
SkiJump Lite- A ski jumping game, quite fun, the Lite version gets dull quickly though
SMB Lite- Super Monkey Ball game for the iPhone. Worth a play, good fun.
Netlog- basically Facebook but rubbish. Don't know why I've got it really.
MMATorch- MMA News App
TwitterFon- Twitter for the iPhone
Sky News- self explanatory
Pole Position- Racing game, very good play. Could be worth buying if I wasn't a cheapskate
iCopter- The classic Helicopter game on the iPhone.
iPity- Mr-T soundboard for the iPhone.
Ebay- self explanatory
ECB Cricket- self explanatory
iBod- Karl Pilkington Soundboard. Best. App. Ever.
Rollercoaster- pretty basic but fun Rollercoaster game
Beatbox Free- Another sound based app, quite fun, the full version would probably be very good
RK_Free- Fitness app, shows you how far you've run, allows for interval timing etc
Paper Toss- Nice fun free app. Worth a play, basically trying to get paper balls into a bin.
Fast and Furious Lite- Another very decent racing sim
Rockstars- Very fun game based around making a rock band
Sky Sports Football Score Centre- self explanatory
iDork Lite- Best game I've played on the iPhone. Superb stuff.
Flick Cricket- Fun little cricket game, not much too it, but good fun
Bike Or Die- Like the old Mousebreaker.com bike game but on the iPhone
Wikipedia- self explanatory
Pop Art Lite- Only used once, turns pictures into Warhol-esque pieces (think it was Warhol anyway)
Biker Blast Off- rubbish.
Mr.Aah...- Quite fun little game, worth a go.
Wow. I have more apps than I thought. Should really get rid of the ones I don't use anymore. A few nice ones in there though. iBod is well worth getting if you're a Pilkington fan, some quality phrases built in.