Sergey Makovkin has released version 0.4 of csDoom, a Doom source port which adds open client/server support (i.e. clients do not all have to start a game at the same time). Sergey states that the only the server source will be available initially, and that the full source tree (including the client) will be available upon the release of version 1.0 (unfortunately, I’m not familiar enough with the GPL’s terms and conditions to state whether or not this is a valid schedule).
csDoom 0.4 Server Source:
csDoom 0.4 Linux Client: