If you tend to tweet on the go, don't worry -- several of the desktop clients reviewed here also have iPhone versions available or upcoming, and there are a bunch of Twitter apps available for other smartphone platforms. For example, there's TwitterBerry and TweetCaster for the Blackberry, and PockeTwit and Twikini for Windows Mobile.
In fact, you can find a very long list of Twitter-related apps at the Twitter Fan Wiki.
If you don't have the time or inclination to wade through that list, you could start by considering one of the following eight applications. Which is best for you will depend on how you use Twitter -- and how you would like to use it.
Web-based HootSuite is unabashedly directed toward those who want to use Twitter for business purposes; its browser tab caption reads "Welcome to HootSuite -- The Professional Twitter Client."
What does it do? Like Seesmic, TweetDeck, and several others, HootSuite works via columns; you can have separate columns for your home feed, for your replies and mentions, for groups, and for any search that you care to do. Hover your cursor over a person's tweet and you get icons that let you mark it as a Favorite or create a direct message, a reply or a retweet. You can monitor several different Twitter accounts (but not your Facebook account), do either a keyword search or a general search, and assign users to groups.
What's cool about it? If you like Web-based Twitter clients -- for example, if you use more than one computer and can't install an independent Web client on all of them -- HootSuite is a good choice. What I especially liked was HootSuite's simple, elegant layout, which made it easy to find and use most of its features -- a distinct advantage when you're not all that Twitter-savvy.
And HootSuite manages to avoid a problem that plagues most other column-centric Twitter clients: If you're trying to monitor more than three or four columns, you end up with a virtual window that stretches well beyond your display's capabilities. HootSuite's use of tabs to separate out sets of columns means that you can easily access -- and see -- a wider variety of searches and feeds.