Its very simple to stalk online users from the Server side, but many of you dont have access to that part [yet]. So in the mean time there are services like Dejanews [http://www.dejanews.com] and other Search Engines like Webcrawler, Infoseek, 411, Yahoo or Lycos. THese search the Internet. Actually it spiders pages gathering every link it can and follows it, and so on, until there are no more links in the page. Links can include email addresses as well.
No if you want to find a particular person or 'online ' person then your first choice should be a search engine. If you know thier email address type it in the search feild. I would suggest using deja news for this (if you want to find out what the user may be into. Dejanews is a web basde newsgroup reader/searcher. You can find out all the newsgroups they have posted to or even replied to. This is the most common way.
ALso some users use other email systems which are in noway affiliated with their servers or anything thereof. Many users online us Hotmail (http://ww.hotmail.com) for free email. THis could make it a bit more challenging. But they are ways to work around that.
When trying other seach engines, try to enter as much information that you know of that person in the search feilds. And then hope you get something back. However it would greatly help if you knew the email address of that user. If you do then you can find out what city and area of that city they may be living in. There are programs that track Internet Servive Providers registered with the NIC. Just search your general '.edu' shareware archive for it. The web also has the 'WhoIs' function that does the same. This can give you the Service Provider and their City/State and Area Code. Though this doesnt mean they live in that area. Some users are parts of major online services like Earthlink, AOL or Mindspring. It may be difficult using the search functions to track them. But finding their web page can compensate.
Im sure I dont have to insult intellegence levels but another way to find out more it so go to there web page. If the user has an email address 'user@server.net' then this could lead you to their homme page. Many Service Providers has areas for users to save data (more specifically-Web Pages). To find out a users, just type 'http://www.the_isp_name.net/~username/'. Play around with the URL if you have no luck. Sometimes it could look like 'http://user(s).the_isp_name.net/~username/'. And the '.net' could be '.com'. Also if nothings still come up, try typing '.home.htm' or '.homepage.html'. Type in the basic default home page names: Here are a few more:
home.html
main.html
toc.html
Also the information retreived from search engines can give you an idea of what it may be. And many times your search could bring up their web page address. And once their you can see more of their world. They may have kids, a dog or a lego set in their toilet. The possabilities are great. Now what you do afterwards is your business. Hopefully its not illegal. Though their is a state who has make it a 'bad' think to spam users. ANd a fine could be encurred. *the idiots*.
Other practical ways include direct interface. You could be talking in a IRC Channel or on a web based chat server. SImply confronting them and befreinding them can make all the difference. THen when you to part from your discussion , just trade emails. Ask them for their web page so you can see it. Ask the what State they live in. Its all up to you.
Now that you understand the general rules of it all, you can go and expand your knowledge.