I just found this out on Twitter yesterday: If you accidentally close a tab you want to get back, press CTRL+Shift+T.  It opens the last tab you closed.

Isn't that brilliant? I don't know many shortcuts, but that one has already burned into my brain. I'm forever closing tabs mere seconds before I remember I really needed them open.

More shortcuts for Mac and Microsoft may be found on Twitter, under the hashtag #smkey.