For years now, i’ve been using a product called Greenshot. It’s basically a free bit of software which enables you to capture specific areas of your screen, but then send them to different locations e.g clipboard, saved folder, email, etc.
Once setup, you can have it load with Windows automatically at startup and with one push of a button, some crosshairs appear and you can drag your selection. I personally have it go to the clipboard as this means no extra windows open and you can paste it into the application you are using at the time.
Unfortunately there isn’t a version that works with Mac as yet. I’ve tried a few others for Mac but have not come across anything as good, and FREE!
To download it simply visit here and click away: http://getgreenshot.org