Yeah there's like bunch of bursae around the knee joint, the only one of which I've bothered to make note of is the prepatellar (inflamed from kneeling too much, "housemaid's knee").
Hmm maybe I didn't yet hit the question you're referring, I just remember a question that implied that I needed to identify the PCL.. so i posted images of what to look for in the ACL and PCL
I can actually help you out with bursitis a bit cause I had that in my shoulder being a former pitcher. A bursae is where the synovial fluid around a joint fills. So inflammation (NOT INFECTION ONLY) leads to bursitis. For some reason people think its caused by infection, but I'm pretty sure throwing a baseball didn't give me an infection in my shoulder.
as for runner's bursa, never heard of it, I think shoulder is definitely the most common location for bursitis.