7capture is a free tool which allows users to take screenshots of any window removing the background image and retaining semi-transparency.
7capture screenshot tool for Windows
No matter what Windows version you are using, 7capture will do its magic: clean rounded corners on Windows XP, alpha transparency in Windows Vista and Windows 7 / 8.
7capture will even capture your theme’s color for even better looking screenshots.
7capture automatically handles complicated transparency problems such as rounded corners and Aero semi-transparent windows backgrounds.
You can export your capture as semi-transparent png, jpeg, gif or bmp formats.
Visit its Home Page for downloading it.