The TextMate homepage probably describes it best:
While TextMate is a fantastic multi-purpose text editor, it is also so packed with features that it can be daunting for first time users. In particular, the bundle system allows for myriad customizations all of which can be bound to particular contexts. In an attempt to help developers just coming to TextMate (and my own terrible memory) I have put together a PDF cheatsheet of some of the more useful general commands available in TextMate.
In case you missed that – get it hereI also highly encourage you to read through the HOWTO’s posted by Soryu, which formed the basis of the cheatsheet:
TextMate is very themable – which leads to hours and hours of wasted time making it look slightly different than it did when you started. The theme below is the result of my wasted hours playing with colors and is called Dawn (it is incidently, in a sense, a reversal of the excellent Twilight theme that is included in the base TextMate distribution):