Pinta is an image editor designed to be simple and look more or less like the famous Paint.NET. It runs on Linux and MacOS X - using Mono, and obviously on Windows too.
Pinta 0.4 was released today with a major change to the user interface which now provides "docking" - you can show, hide, resize and undock various elements in Pinta, to suit your needs. Pinta is now fully translatable and the latest 0.4 version comes with 18 languages (some are not 100% finished) - you can help translate Pinta via Launchpad.
Other changes in Pinta 0.4 include new supported file formats like gif, tiff, bmp and OpenRaster (compatible with Krita or MyPaint), font preview for the text tool, better GNOME integration and lots of bugs fixed.
Download Pinta 0.4 - packages available for OpenSUSE, MacOS X and Windows.
Update: Pinta 0.4 is now available in its PPA and you can install it in Ubuntu Lucid using the following commands:
sudo add-apt-repository ppa:moonlight-team/pinta && sudo apt-get update sudo apt-get install pinta
Use the commands below to convert the OpenSUSE .rpm to .deb and install it for other Ubuntu versions:
wget http://github.com/downloads/jpobst/Pinta/pinta-0.4.noarch.rpm sudo apt-get install alien sudo alien -i pinta-0.4.noarch.rpm
Note: the logo in the screenshot was made using the GIMP, not Pinta =)