Paint.NET equivalent for Linux?

Pinta looks pretty good.

Pinta is a drawing/editing program modeled after Paint.NET. It's goal is to provide a simplified alternative to GIMP for casual users. It is currently early in development.

KDE comes with KolourPaint, which is essentially exactly like Paint for windows. If you want a little more power, KOffice includes Krita, which I've really liked. I'm sure there are also some GTK-based apps if that's your preference but I've definitely found both KolorPaint and Krita useful, so you really should check them out.

Looks like paint.net can run on Mono the .NET implemetation for linux. The linux version is called paint-mono. Not sure you can find readily available binaries for that, but maybe you don't mind compiling it from the source code ;)

You may also want to check out inkscape. It is a multi-platform vector image editor. Vector editing is a little different than pixel-based editing, but I find that it to be preferable -- you can move items around, change colours, fill styles, etc, and, when your done, export it to an image.

See also the Bitmap Editor? thread on the ubuntu forums.