Mac OS X: Windows 7 added a great snap feature for quickly resizing windows to various portions of your screen. Free menu bar utility ShiftIt brings the same sort of functionality to OS X in a keyboard-friendly package.
The app is pretty simple to use. Once installed, it adds keyboard shortcuts that quickly move and resize the active window to the top left/right, bottom left/right, left/right/bottom/top half, or centre of your screen. Hit Opt+Cmd+F to go fullscreen (like Windows' maximise). You can refresh yourself on any of the shortcuts by clicking the Shift drop-down in the menu bar.
ShiftIt is a free download for Mac OS X only. Pre-Windows 7 users, check out previously mentioned AeroSnap to get similar functionality in Vista or XP.