Splashtop works well locally, but requires port forwarding over 3G.
Teamviewer is free and works from everywhere without specific setup, however, I don't like how it handles the mouse, you need to move the courser where you want to click, with Splashtop, you just tap and it interprets it as a mouse click in that location.
Further apps to consider, but are more expensive:
iTeleport
Logmein ignition