Ctrl + Click a layer - Load selection of the clicked layer
Shift + Ctrl + N - Create new layer
Ctrl + S - Save document(should be used often, I learnt the hard way! >_<)
Ctrl + E - Merge 2 layers together
Shift + Ctrl + E - Merge all visible layers
Shift + Ctrl + M - Export document to Dreamweaver (I always hit this one when I create a new layer)

I know a lot more than that but most of them I just do them without even noticing so I can't remember exactly what the keys are.