Twhirl is a twitter desktop client based on the Adobe AIR platform. It's still in beta stage but already includes lots of interesting tools.

These are some of Twhirl's features:

• Automatically fetches your friends' status updates, as well as replies and direct messages for you, into an all-in-one timeline
• Color-coding different types of messages
• Connecting to multiple Twitter accounts
• Keyboard shortcuts
• Sys tray / dock icon support
• Visual and audible notifications on new tweets, configurable and with preview
• View, add and remove favorites
• Shorten long URLs (using snurl.com)
• (Un)follow users from twhirl
• Automatically checking for twhirl updates
• Changeable color schemes