You can install GTK apps like leafpad, but they look ugly. Change the GTK theme with: http://hints.macworld.com/article.php?story=20080309171359879
You can use the mac4lin gtk theme, see: http://pepijndevos.nl/2010/04/mac4lin-on-mac
Execution with quicksilver http://www.simplehelp.net/2007/05/09/how-to-install-linux-applications-in-os-x-a-complete-walkthrough/#quicksilver