Anybody who follows me on twitter will know that @rikhepworth is by no means a prolific tweeter. However, I do follow a number of people around the planet, and in addition to the ubiquitous Tweetie2 on my iPhone, I have found two clients to be useful and reliable.
The first is Tweetz, from Blue Onion Software. This is a great gadget for the Windows 7 desktop (or Vista Sidebar). The UI is simple and extremely usable (I love the way I can scroll the history for older tweets) and it makes posting a breeze.
The second reflects just how much I live by Outlook and the resulting ability to search and collate unread mails, blog posts and now tweets. Twinbox from TechHit allows you to tweet directly from Outlook and incoming tweets are collated by sender. No integration with the Office 2010 fluent UI but the add-in works, and there is a 64-bit version available as well.