Windows: We've covered a lot of screenshot apps in the past. If you use Dropbox for most of your working documents it may make a lot of sense to use CloudShot, the screenshot app that automatically uploads to a specified folder on your system, including Dropbox folders.
The small and free program also includes most other request screenshot tool features: the ability to take screenshots of a specific screen region, the ability to take screenshots of just a window, menu, or tooltip. You can use it with multiple monitors, and it includes a history with the 10 most recent screenshots.
The Dropbox integration, multiple monitors, and history features push this one to the top of the heap of screenshot tools for me. If you routinely take screenshots I recommend giving it a try.
CloudShot is a free download for Windows at the developer link below.