Twitter For iPhone.
I have tested a few applications for iPhone that will help me to keep up with my twitter needs. I was not very happy but managed to to get along. Then came the Twitterrific, now I enjoy twittering on my iPhone.
Twitterrific is free to use if you are willing to put up with an inline advertisement at the top of the timeline. If you find the ads intrusive or want to support the developers, you can purchase Twitterrific Premium via the App Store which removes the advertising.
The twitterrific brings support for multiple twitter accounts, searches, trends & filters, new visual themes and layouts, conversation threads, a greatly improved posting interface, marked tweets and more.