Short-version of Settlers of Catan
We've tried to replace the robber with a more benevolent figure (we used a LEGO Dumbledore microfig but you can just reuse the robber figure and call it, say, a magician)  


When you roll a 7, move the magician to the tile you want
If the number on which the magician sits is rolled, all tiles overlooked by the magician get double the ressources for that turn


We came up with that because we don't like the robber that much, and it sped up the game quite a bit. Now, I suppose any house rule which gives more ressources would shorten the game length, but with that one is slightly strategic in nature.  

Of course there's also the bog-standard way to change the duration of the game: instead of 10 victory points, aim for less (8 for example). As long as you agree before starting the game :-)