Some tricks of the trade

Repeats

A number preceding an operator or command - e.g.

3dd	Delete 3 lines
10h	Move 10 characters right
10>>	Indent 10 lines (a little unusual)

~	Change case of letter under cursor
ctrl-^	Flip to previous edited file
J	Join two lines together
u	Undo.
U	Undo all changes on current line
o	Insert a new line under the current one and enter insert mode
ctrl-]	Jump to definition of function (tag - use ctags to generate index)
xp	Swap two letters round
A	Append at end of line