Number keys = numbers
Space = Enter/OK/Select (at least for my Verizon Motorola set top box)
CTRL+L = jump to last channel
SHIFT+> (greater than symbol on the period key) = SlingPlayer volume up
SHIFT+< (less than symbol on the comma key) = SlingPlayer volume down
ALT+SHIFT+L = Letterbox 16:9 crop
ALT+SHIFT+N = Normal 4:3
ALT+SHIFT+A = Anamorphic
ALT+ENTER = Full Screen Toggle
ALT+. (period) = SlingBar on right
I'm sure there's documentation for more but that's what I know off the top of my head and use relatively often.
If you leave the mouse pointer parked over the on-screen remote's buttons, a tooltip should pop up that gives you the keyboard equivalent.
If you're not seeing that using the Web Player, it works in the full SlingPlayer app.
Most of the keyboard equivalents work in the Web Player and the full SlingPlayer.
Hope this helps,