Rather than doing a review or guide for custom maps, I thought that it would be better if I gave you a few links to sites that host and revolve around custom maps. So here are one or two links that you might already know about, and a quick description of each site:
This is a good website that is dedicated to helping mappers create new maps for the game that we all know and love, not only do they provide a place for you to showcase your map, there is also a large community, and resources and links to tutorials to help you on your way. They also host competitions from time to time and were responsible for producing the back to front maps.
I don’t know too about this website, although it does seem to be still running, the last map released on it was done so just 2 days ago!
Some links to the more famous or popular community maps