There are many useful hotkeys but here are the ones I find I use the most (not including what Philip wrote)
:
Shift+g : snap (like drag [ctrl+t] only it snaps to points)
F2 : center the object
~ : bend
[ : shear
Shift+d : Subdivide
q : set surface
f : flip polygons
ctrl+k : Spin quads
a : Fit all the mesh into the window
shift+a : fit all selected into the window
- : hide selected
= : hide unselected
\ : unhide