Clipboard2Image is a free tool, initially developed for Tabula Rasa (and its lack of screenshot taker, until a recent patch). When Clipboard2Image is running, if bitmap data is present in the clipboard (when PrintScreen key is pressed), its content is written to a file, quite simple...
Click the 'Select Path...' button and select the local folder in which to save images. The image format is selected with the 'Output format' combo box. Available formats are :

Clipboard2Image (tr_c2i) interface
Run and install Clipboard2Image (tr_c2i)
Clipboard2Image was tested with Tabula Rasa (before patch) and Unreal Tournament 3. It also works with World of Warcraft, but this makes two screenshots :)

Tabula Rasa screenshot

Unreal Tournament 3 screenshot

World of Warcraft screenshot
If you like this free tool or found a bug, send me an email ;)