There is also remoku.tv (website). It runs on most modern browsers (Chrome 11+, Firefox 5+, iOS4+(Mobile Safari), IE 8+, etc.).
If you need help you can visit help.remoku.tv for setup instructions, tips, and to report any issues.
If you use chrome, it is easy to make an Application Shortcut for Remoku, which will launch it in it's own window. Browse to Remoku.tv, then click the wrench icon near the top-right of the Chrome window, then under the Tools menu, choose "Create application shortcuts" and choose the locations you want the shortcuts to appear on your computer (e.g. "Desktop", "Start Menu", and/or "Quick Launch Bar".
Besides the basic remote functions that hardware remotes have, Remoku also can send text directly, and features a few other goodies.