Back in June David Canar from the Qyoto project emailed me pre-announcing the launch of Qyoto. Qyoto is a Mono/.NET bindings for TrollTech's Qt4 library:

You can:

• Watch Richard Dale and Arno Rehn presentation on Qymono.
• Install Qymono
• See Hello World as written in Qymono.

This is another toolkit that you can use to develop cross platform GUI applications.