Handy keyboard shortcuts

[Rahky]

Alt-click: examines objects.
Ctrl-click: pulls objects.
Page down or Page up: changes hands. Not sure which one.
Mouse wheel clicky thingie: Changes hands. Sometimes unresponsive.

Adding onto this, assuming you have a numpad (I don't know whether or not you can do it without one):
Home [7]: Drops the item in the selected hand
Page Down [3]: Use the item held in the selected hand. Functions the same way as clicking on the held item.
Page Up [9]: As Tteckk mentioned, switches hands, though usually need to be pressed quickly, as if held down it will switch back and forth between hands rapidly.