The iPhone's multitouch system, GPS and 3G connectivity make it the perfect portable Twitter device, and there are several great programs that take advantage of it.
Twitterific is our favourite, purely for the clean interface and the excellent way that it distinguishes between different post types, but others include Tweetie (with an interface reminiscent of the standard iPhone SMS system) and the GPS-enabled Twinkle.
Twitterific doesn't offer the obvious 'killer' iPhone feature of showing you posts made near to your geographical location – something that can come in useful at events, if not necessarily while walking home – but it handles everything else with aplomb.
It's available in two forms, an ad-supported freebie version and the £6 Premium version.
The only real annoyance is that there's no way of telling it whether to start the scroller on the latest posts or snap back to where you left off – a real nuisance if you use it to catch up on posts every now and again rather than as your primary Twitter reader.
Both versions have their use, but be prepared to waste a lot of time scrolling to see what's new.