Shortcut for backpack to hotbar?

[Sladus]

Is there a shortcut to put things in the backpack to an empty space in the hotbar?

[I Kinda Fail]

Shift+click? I think that's what you're asking for. You can also use Tab to cycle between your backpack's rows.

[PhailRaptor]

Tab will cycle through your (up to) 3 rows of backpack space, you can use the mouse wheel or the hotkeys to select a specific item in the current row.