Since I can't give a short comment - here a long answer.
First of all: What lorancou said!
To go more into detail: What do you need axactly?
Chat - check some common protocols (like IRC) ... BTW a good hint for the gaming relevant things
But I guess it is about gaming relevant issues, so there some general hints I can think of boosting the performance
- Minimize traffic
- It is enough to send a movement vector from time to time (try the exact duration)
- In most cases UDP is sufficient - no ack necessary
- Minimize number of server requests
- let the client do a lot of the calculations (if you share the random seed and the stats, it is enough to transmit the actions, every client can calc the exact values and from occasionally there can be a check)
- you don't need to send the action instantly, it can be enough to send them bundled from time to time)
- Partition the area, similar to Occlusion Culling in graphics
That are some things I can think of right now w/o much thinking, if you think hard enough you get a lot more information.
Maybe I'm missing the point of your question - in this case you need to clarify.