I wouldn't say I have mastered Ruby but i am trying to get there and there are two languages that intrigue me now days. One is good old C and the other is Erlang. Why? Well, you can basically use both to write a server and I have been kind of interested in not actually writing my own server, but just getting involved in one such project.
Erlang does have some merits over C. First of all, it promotes concurrency. No shared memory, just message passing. It is so much helpful to write a multithreaded program since I don't have to worry about shared memory and trust me writing multithreaded code is hard. Anything to do with shared memory is hard since you never know when CPU cycles kick in and break your entire program. Anyways, I am still indecisive as to what shall I learn next but I am so much looking forward to it.
I'm out of leguae here. Too much brain power on display!