I was thinking about the recent post for making a collage. Well this is a photo-montage rather than a collage in the strict art sense.
Mashup: Very easy to use, drag-and-drop works, resize / reposition images by dragging, add borders, text, rotate, all WYSIWYG.
Also from the same author,
Fotoxx: Lots of tools but in particular the warp tools are interesting, more like the gimp 2.7 cage tool but can work on irregular selections or the whole image.
Web page is http://kornelix.squarespace.com/
The packages have recently been updated and are noticeably quicker than previous versions. Providing you have a recent ubuntu (or derivative) or a fedora there are deb's and rpm's. The new versions did take a little bit of persuading to work in my PClinuxOS.
(edit: correct typos)