While Janetter claims to be the best Twitter client on the Web, the statement would have be probably revise to the one that it is one of the best and one of the most flexible Twitter multi-platform clients available. The application came to public early this year and is developing fast to a good product based on the users’ recommendations.
There are multiple attractive features available, such as: multi-column view, support for multiple accounts, 27 different themes, customizable fonts and display formats, wallpaper, notifications, auto-complete, keyboard shortcuts and language translation. If you're using the Windows version you can even create your own themes in HTML.
Official website: http://janetter.net/