I know a couple of webmasters and they have told me that they can track anyone that visits their sites. There is simple code that anyone can use that reveals the I.P. address of other people posting on a forum. There are databases that show billing addresses of those I.P. addresses.
You're only partially correct. A webserver has the ability to know which IP address you are using. However, in almost all instances, individuals do no have their own IP address assigned. The Internet Service Provider dynamically assigns the IP address from a pool of addresses they own. You can track the IP address back to the Internet Service Provider doing what is called a WHOIS search.
The ISP will keep logs for a certain period of time of who is assigned what IP address at any particular time but these are not be accessible publicly. In order to get these, you have to subpoena the ISP's records and hope they haven't been discarded when your subpoena hits. You then get the physical address from the ISP.
Now, if you are the rare individual who has a stactic IP address assigned to you by name, it is possible to do a WHOIS search and get a physical address (at least a billing address). If you don't know if you have a static IP address, you don't have one.