Hello there! Long time ago that I was “active” here. 
I would like to ask if anyone knows about or has a document which describes the Quake3 protocol, better the current version of the ET protocol? I’ve been searching a lot on the internet and couldn’t find any, maybe some of you have such a document or an idea where to obtain it
Thanks in advance. 
Of the actual packets on the wire, I’m not aware of any. There is a Display Filter for Q3 in ethereal http://www.ethereal.com/docs/dfref/q/quake3.html but it doesn’t give you much information,and is perhaps incomplete or wrong. Source here: http://anonsvn.ethereal.com/viewcvs/viewcvs.py/releases/ethereal-0.10.9/epan/dissectors/packet-quake3.c?rev=13108&view=markup
From my observations, most of the contents of the packets are obfusacted (for good reason), so understanding it isn’t just a matter of grabbing the packets off the wire.
There are some documents describing higher level stuff (e.g. from a gamecode POV), for example the documentation included with Neil Torontos unlagged code, which you can find at http://www.planetquake.com/alternatefire/ (note, I’m talking about the documentation included with the actual source, not the ‘unlagged FAQ’)
The Q2 engine source is available and, being an ancestor of Q3, might give you some insight.
I have already captured packets transmitted between client and server using ethereal, and I couldn’t understand the content at all (not the UDP packets).
Thank you for the links and your help. I’ll take a look at them.