"POPULAR"
Real Server is a Real map server(which is based in TFS) which most people download and run to get money, allisow.. err, I don't have an idea about allisow, but IIRC, it's based in TFS(or OTServ, can't really recall).
As you can see, there's 8,477 players online in servers that use 0.4 and there's only 3,616 in Real server 3.1 and 1139 in Real Servr 3.0, because all the "real servers" are just download and run Cipbia maps, which people sometimes hate and would rather get in a server that has been "edited".
0.3.6 is a great release, although people do not use it because it's 8.54.