XnView Software

It'd really be good if we could create multipage tiffs/pdfs/etc either by pasting an image into a new page/frame (ie, hit the print screen button, then ctrl+v (or whatever shortcut you want to use) to paste the captured image into a new page in the multipage image), or by combining currently open tabs into a single multipage image (ie, tools->multipage->create from tabs->format selection dialog).
I think the second option would be the best one.