SittingMod

Press 'N' to sit down on nearest object, press 'M' to sit down.
If you're already sitting, press 'N' or 'M' to stand up.

You have to install Aru's Scripthook, a DLL/Asi-Loader and for use in Multiplayermode Alexanders Filecheckfix.
Visit http://www.gtaforums.com for further Information