Alright, so most of this is just a repost of Tim's invaluable stuff but there is new stuff in here as well.
Joining your own server
If you have the server on the same pc you are running your client on,
then you'll need to do the following
Add "verify-names=false" to your "server.properties" file
Restart the server (if it was already started)
Join with http://www.minecraft.net/play.jsp?ip=127.0.0.1&port=
The above will work if you're on the same computer to host as to play. However, if you have a host and a client computer on the same LAN, then you need to use the following adress: