I'm looking for a simple, fast and intuitive tool for sketching/drawing/painting in Ubuntu/Linux.
I have Gimp installed and I like it, but I'm looking for something simpler and faster, to quickly draw/paint ideas etc.
For posterity, I recommend KolourPaint. It's an MS Paint clone (i.e., as prior to Windows 7) from KDE, but unlike other MS Paint clones (I have tried them all) it is actually as intuitive and quick as the original. Its user interface is a lot less menu-y than Pinta, and it loads far faster than GIMP, Krita, or Inkscape.
It doesn't support layering but for quick and dirty stuff I don't find this to be much of a problem.
Inkscape is a vector drawing package which I find useful for quick drawings and diagrams. It is in the repos and I believe it can easily be installed with
sudo apt-get install inkscape (I'm on Debian, but the install command should be the same.