Lego Half Life 2 is a mod that does exactly what the name suggests. Created by Not Dave or Daniel, it takes the classic first-person shooter and turns it into a LEGO game, complete with bricky inhabitants.
It doesn’t overhaul the entire game, so don’t go expecting bricky vehicles and bricky landscapes and bricky buildings; the only things being swapped out here are the characters. But given most LEGO games leave a lot of the landscapes “normal” anyway, the effect is still pretty good!
There are some perspective issues, of course. Swapping out full-size humans for tiny little yellow men and women (and aliens) will do that to your viewpoint. But it looks manageable, and more importantly, it looks very funny.
You can download the mod here.