Here's a little something that I found out today, pressing Ctrl, Command (Apple) and D in Safari (OS X, not sure about Windows) gives you a mini popup window with a dictionary defintion or a choice of alternatives from the Thesaurus:

Pretty neat eh? Also if you enable the debug menu in Safari:

defaults write com.apple.Safari IncludeDebugMenu Yes

Then you can also produce a list of all the keyboard shortcuts in Safari - for those that are Terminally(sic) challenged you can download a PDF of it here