Meld is this awesome diff and merge tool that earns plenty of coolness points for all the amazing features that it provides [three way merges, directory diffs, tabs for multiple simultaneous diffs, syntax highlighting, user defined editor, cvs/svn plugins, all those pretty curves in the diffs ;) etc.]. It is by far the best diff and merge tool that I've come across and shows diffs accurately where other tools like xxdiff, vimdiff etc. are not able to do anything sane. Infact meld happens to be the only reason why there are some gnome libraries on my not-so-studly desktop.
If it bothers you that meld doesn't have a 'Save As' entry in the File menu and the toolbar, try this patch. The patch is not Debian specific and can be applied upstream.
Update: Part of the patch has been merged upstream and should be available in the next version of meld.