key commands you should know

Command-O Open
Command-S Save
Shift-Command-S Save as
Command-P Print
Command-W Close Window
Command-T create new tab (esp. useful for browsing in Firefox)
Command-I Get Info
Command-D Duplicate
Command-Delete Move to Trash
Command-E Eject
Command-F Find
Command-Z Undo
Command-X Cut
Command-C Copy
Command-V Paste
Command-A Select All

And there are lots more, check it out here.