One of my favorite web sites is the GPSFileDepot. It's a great source for free map data for your GPS receiver, assuming you have one that lets you load your own files. The home page maintains a running list of the latest data files that have been added, along with the most popular. You can also search for available maps by location, and the tutorials and forums are both very helpful.
A while ago, I downloaded the California TOPO map for my Garmin, and I've found it to be very detailed and accurate. While you cannot navigate by roads or trails like you can with Garmin's $99 TOPO product, I find that the TOPO file in the GPSFileDepot library includes many, many more trails than Garmin's 1:24,000 TOPO version does. In fact, I have yet to find a trail missing from the map.
The maps available for dowload here are all free because they have been created and donated by generous users. As of this writing, there were 187 custom maps available and there had been over 91 thousand downloads. Take some time to look over the site. You'll be glad you did.