its not all by myself though.
often, indirect contributions can be much more valuable than direct ones. the simple reason being that it forces you to actually understand the whole instead of just the interfaces.
and I do get quite a few indirect contributions still, even if its someone reporting a bug (onemanclan has reported a few), or an admin saving giving me a copy of a saved game when some bug struck (bigfoot), or just trying out the weird and whacky features just to see if they actually have a chance of being more than just a gimmick (gb often does this, finding fun and whacky bugs along the way), or just existing APIs in other engines/mods that give me a testcase(xonotic is one demanding test case...).
And yes, I do sometimes rip code directly from other engines where I think it'll save me time... though I do at least try to understand enough of it before doing so.
the ragdoll code, for instance, depends upon the third party ode library. it uses code based on DP's MOVETYPE_PHYSICS stuff (and thus should interact properly with such entities, including solid stuff like the world). The fact that I wrote a little code around it to parse a .doll file, generate physics bodies+joints and animate the mesh by tracking the physics bodies is somewhat insignificant in comparison to the length of time that it must have taken to write ODE in the first place. Incidentily, I hate matrix maths.