Both from the same developer. One is called "SizeUp" and I've been using it for a few days now. Very useful program, especially if you find yourself wanting to quickly compare two webpages side by side. It uses keyboard shortcuts although you can also select it from the menu bar. I haven't tried "Cinch" but it looks to be more mouse-centric if that's your preference.
Check 'em out.
http://www.irradiatedsoftware.com/
Check 'em out.
http://www.irradiatedsoftware.com/