There are others like GTK#, WX.NET, QT#, FLTK.NET. I'm assuming by
lightweight you mean simpler to use (instead of lightweight like java's
swing). None of the other gui toolkits are simpler to use than
Windows.Forms though. They do work on other platforms besides Windows.
GTK# starter resources:
http://www.go-mono.com/gtk-sharp.html
http://www.oreillynet.com/pub/wlg/5390
http://www.onlamp.com/pub/a/onlamp/excerpt/MonoTDN_chap1/?page=last
One thing that makes it harder is the programming language. You might
check out boo, which is simple to use like python, but works for .NET:
http://boo.codehaus.org/
Here's a sample program:
import System.Windows.Forms from System.Windows.Forms
f = Form(Text: "Hello")
b = Button(Text: "Click Me",
Dock : DockStyle.Fill)
b.Click += do ():
print "clicked"
f.Controls.Add(b)
Application.Run(f)