Afloat is a handy little plugin for your Mac that gives you more control over all the windows you've got open. Once installed, Afloat adds some new options to the Window menu in most applications, allowing you to do some neat tricks. Here's a rundown of the options, which almost all have keyboard shortcuts:

The keep afloat command assigns a window to stay on top of all others. Alternatively, you can pin windows to the bottom of the stack, at desktop level. Window transparency is adjustable using either a slider or keyboard shortcuts. Afloat allows moving and resizing from any part of a window if you hold down cmd+ctrl - just grab an edge to resize, or drag from anywhere to move.

If you use Spaces, you can use Afloat to make a window appear across all Spaces, so you won't lose it when you switch. There's also a neat overlay feature that makes your window transparent and unclickable, so you can use it as a guide while you work with whatever's underneath - I can think of some fun design applications for that one. Afloat also has a hotkey to locate the file associated with the current window, which is something the OS X Finder could probably use as a standard feature.

It's not that Afloat adds essential functionality to the OS. In fact, it's probably overkill for most people, but just about every Mac user should be able to find one feature to like in this handy little plugin.