For Windows only: The guy at PotableApps.com have released version 1.0 of Lightscreen Portable. Lightscreen is a lightweight open-source application that has only one purpose, taking screenshots. With no in-built image editor, your screenshot files can be taken with a hotkey, and saved in the JPG, BMP or PNG formats, using a nomenclature of your choice.
Lightscreen Portable 1.0, adds Windows 7 support and has a multilingual installer. It’s packaged in PortableApps.com format for easy use from any portable device and integration with the PortableApps.com Suite.