Putting aside the discussion of ethical aspects of such actions for another time, this is a great way to find out more about yourself, lose your best friends, and make yourself subject to sudden attacks of conscience.

I first came into contact with this kind of activity while talking to someone whose name I promised not to disclose. This person pretended to be a completely different person on ICQ while talking to one of our mutual friends. This began an innocent joke, but the person was drawn into it, and abandoned their plans to reveal their true identity, instead opting for the tasty bits opinions they would not otherwise have ever heard of (such as, for example, that the other person had a crush on the impersonator in real life). Highly embarrassing, and inconvenient, since they had to let every one of their friends on the plan, so they would not accidentally disclose the innocent fact of the impostor having an ICQ adress to the victim.

In another instance, one of my friends, Morte here on everything, pretended to be someone else to necromancer, resulting in a long string of words which summed up, basically, to "All my friends are assholes". Now Morte intends to ^6Hsdf4DCENSORED BY MORTE. But I'm getting off track here.

If you intend to perform this feat of deception, here are some useful hints:
0. Do not do this.
1. Do not-Not-NOT do this if you're unsure of your qualifications as an actor. You will be discovered, and the consequences are usually negative.
2. Above all else, be careful! Do not mention the names of your mutual friends, keep to the writing style you picked for your alter ego, make sure your legend is consistent.
3. Do not be too insistent asking for gossip about your real self.
4. To quell their suspicions, try to appear online at the same time as your alter ego, using multi icq hack, or two computers, the latter method having the advantage of a possibility to have different IP addresses for your egos (for the benefit of the more suspicious of your friends)

Update: Check out the node on Jaymz

The Penguin Rears Her Ugly Head