These are listed best first, if the app is only relavent to UK users like myself it will have #UK after
The Ebay iphone app is awesome, with only 480x320 pixies Ebay have managed to create a user experience that blows away it's own website. Admitedly the the Ebay website does not set the bar very high. The UI is clean and functional search is easy and it remembers your searchs. Once you have found the item you wish to bid on or buy it takes only 3 clicks to enter your bid and one more to confirm. For me this is the app that all other shopping app's should aspire to.
Tube Map - #UK
A great tool to help you navigate Londons Underground
Great tool for techies allows you ping, tracerout and telnet.
One of several twitter clients available very handy for catching up on twitter.
No explanation necessary