I just read about DiffDoc on Buck Hodges' blog. This thing's amazing - it shows differences between all sorts of businessy document formats - MS Word, Excel, PowerPoint, Wordperfect, PDF, RTF, and HTML
Wow! I don't know how many times I've been handed a new version of a software spec with no clues as to what has changed between versions, and this would have saved me quite a bit of grief.
[Source: Buck Hodges - How to diff Word documents ]