How do I quickly position two windows side by side?

There are good solutions for both Mac and Windows that I know of:

Windows 7

This feature is built in, and it's called Aero Snap. You can use the following keyboard shortcuts to get the behavior you want:

• Win + ← moves the current window to the left half of the screen.
• Win + → moves the current window to the right half of the screen.
• Win + ↑ maximes the current window.

Windows XP and Vista

GridMove allows you to setup regions to snap windows to via a shortcut key. I mapped Win + 1 to move a window to the left half of the screen and Win + 2 to use the right half.

AeroSnap makes Windows 7's native keyboard shortcuts available in Windows Vista and Windows XP.

Mac OS X

Cinch, SizeUp (both unlimited free trials) and TwoUp (discontinued – available here) from Irradiated Software all accomplish this task perfectly. SizeUp adds some extra features, including multiple monitor support.

Answer for Windows as explained on the old new thing:

In the taskbar, click the button for the first window you want to position, then hold the Ctrl key and right-click the button for the second window. Select Tile Vertically. Bingo, the two windows are positioned side by side.

As mentioned in a comment below, you can also hold Ctrl while you click an additional taskbar button and finally right-click a third one. In this way you can arrange 3 windows at the same time. More than 3 windows also works but results in a grid layout.

Mac OSX

Two excellent, free tools:

• Spectacle is ultra-simple and lets you setup shortcut keys to all the typical window placement actions (fill left half of screen, fill top half, full screen, center, etc).

• BetterTouchTool is very full featured and can do these keyboard shortcuts as well, but also lets you click and drag windows to "snap" to different regions of the screen. (And it has tons of other features, since it's original purpose was custom touchpad gestures).