It's long bugged me that there isn't a good source for viewing and printing seamless USGS topo maps online. Terraserver and MyTopo both have great nationwide coverage, but the quality of their scans can suffer, especially at printed resolutions. Web-based map viewers also tend to fall short when it comes to things like datums, UTM grids, etc.
So, using the data available at http://atlas.ca.gov/quads/
, I went out and created a set of map layers for California. It was a real pain and I probably never would have started if I'd known what was involved, but the final quality is great. I also created a map viewer that lets you overlay a UTM grid, switch between WGS84 and NAD27, make full-page prints, draw routes and waypoints, and transfer data to/from a GPS. You can browse the map layers at http://caltopo.com/map.html
, or sign in to create a saved map, which lets you create routes and share the data with others. A very basic user guide is at http://www.caltopo.com/about.html
The obvious benefit is the ability to load a few tracks and print a map (I've mad some nice 20x30 PDFs that are going on my wall once they get printed), but I'd really like to see people use the site to create maps that they can share with others: local bike trails, backcountry ski routes, named cliffs at a climbing area, etc. You can share a saved map with others by giving them the url of the map page, although you can also make a map entirely private or give others write access. As an example, here's a map I created showing a loop hike for Price-Aggasiz in Desolation that I like taking people on (trails in black, scrambling in red):http://caltopo.com/map?id=FF37706C
In addition to USGS topos, the USFS road maps are super useful for navigating the maze of logging road sometimes required to reach a trailhead, although they're a bit dated. For an example, compare this map of the road to Shasta's North Gate trailhead to the USGS topo: http://bit.ly/rJrYud
All of this originated from a project I've been working on to host web maps locally for Search & Rescue, and the code powering it is the same, with a few UI tweaks to hide the SAR-specific stuff. As a result, parts of the UI are klunkier than if I'd written it as a standalone map viewer. I'd also like to add some additional map layers (contour lines that you can overlay onto aerial images, current USFS roads from their shapefiles, historical topos from the 1800s), but I've spent a bunch of time getting this ready for public consumption and need to give the SAR part of the codebase some love for now. Suggestions, criticisms, etc. welcomed in the thread or through PM/Email.