I have my own server to host my main site and have been buying and hosting a couple of sites for friends and a local club. My main site has a reasonably generic name and after reading about people operating lots of sites, I thought about expanding my collection. The only downside was the cost registering names etc.
Something in the back of my mind said you can use sub domains instead.

So www.mysite.com can also have www.also.mysite.com and www.thisis.mysite.com

I added a sub domain name to the server and updated the NS record. Suprised to find that it was as easy as that! The site sprang into life - no waiting for registration, instant new site names and now an unlimited potential to create lots of sites for free.

So question - am I missing something? Any problems with search engines / users? Do I have to worry about trademarks - eg www.cnn.mysite.com?