Hi,
There are several plugins on which you can play online.
There is the RBRCzech plugin as mentioned, RSRBR have a good mods system and a fine working online function, and the Hungarian rallysimfans community have a plugin.
Each of these plugins have their disadvantages, and are not working together when put in one folder. For that I would propose (question read earlier in this topic) to leave Czech RBRTM plugin in your original RBR install, copy the complete install to another folder and install RSRBR after removing RBRTM, and rename to xxx/RSRBR for example. Then you can direct the RSCenter interface to start this copy of RBR, works fine for me.
For myself I use RSRBR mostly nowadays, and organise the RDO and MRK (unofficial) championships via this system.
I read someone saying Czech is the best plugin, I don't agree on that. It starts with the fact you have to pay to be able to post a rally, pay to drive certain stages, only to find that the fast drivers mentioned who are playing RBR 24/7 will always beat you. I think it's not right to advertise on a neutral forum anyway....but that's just me.