Windows: You can find entire application suites focused on adding functionality to multi-monitor setups. MonitorSwitch focuses on a single feature — slinging applications from monitor to monitor — and does it in an unobtrusively and effectively.
MonitorSwitch works on all versions of Windows but it's the most unobtrusive on Windows 7. For reasons not clarified in the program documentation the two small red arrows for moving the window to the previous and next monitor are gone in Windows 7 (but present in Windows XP) and the only access to the process is through right clicking on the application titlebar — as seen in the screenshot above. This adds an extra right click to each move but it keeps the title bar unmarred by third-party buttons which often fail to mesh with your theme and the over all appearance of the OS — in fairness they don't look horribly out of place with the default Windows XP theme, an OS where they actually make an appearance.
MonitorSwitch is freeware, Windows only. Have a favourite tool for enhancing multiple monitors? Let's hear about it in the comments.