Base Maps for GPS Receivers
I currently use a Garmin Oregon 300 GPS Receiver and find it be a terrific device. One of the best features is the ability to load custom maps into the device. There are numerous commercial map products available from Garmin, but there's also a large number of free alternative basemaps available for those that are willing to search the web. (These maps may work for older eTrek/Legend series of receivers, but you may have to experiment a bit.)
The OpenStreetMap.org site offers a "Wikipedia of maps" - anyone can make edits, additions, or deletions to a worldwide map, making the underlying map product very useful (depending on the degree of user interaction, just like on wikipedia). There are a number of free tools that can enable you to convert the OpenStreetMap data to a format recognizable by your GPS receiver but, at least in my experience, these are complicated and confusing to use; besides - a number of people (or services on the web) will do this for you! Most of the free Garmin base maps are based on the OpenStreetMap map data but many have been augmented by their authors to include more details (perhaps from other data sources like trail databases) or improve icon/color selection for GPS receivers.
Most of these maps are "routable", meaning that the BaseCamp software and your GPS unit can follow roads or trails to the destination rather than simply drawing a straight line. In theory, you may also be able to use these maps in car GPS receiver units as well (since I believe Garmin uses the same map format for both devices) to help you in driving around where ever you are going. As of 2011, none of these free maps have elevation data (so that you can plot elevation profiles) as that seems to be a proprietary Garmin data format that no one has reverse engineered yet. If you need to determine elevation profiles for anything, just use BaseCamp to export your route to Google Earth and use its excellent elevation profile tools instead.
For various trips I've done, I've scoured the web looking for good maps for my GPS (after trying and failing to create mine). I found a number of excellent maps for my Garmin (and there are some for other GPS receivers as well) and they're all linked to below.
Note that for the Garmin receivers, there are two "flavors" of map images. One is a large (typically a few hundred MB) .img (such as the standard gmapsup.img) file that you can copy directly on to your GPS receiver and view on the receiver - this is really handy, but doesn't display in Garmin's BaseCamp software unless you have your receiver connected (and even then, it's fairly slow to load since it is being read from the unit itself). The other format places hundreds of smaller map tile img files on your hard drive which cannot be directly transferred to your device by copying. These have to be "installed" onto your machine for BaseCamp to see them, but after that you can use the Garmin MapInstall program (which comes with BaseCamp, but) you may have to dig around in "C:\Program Files\Garmin" to find it) to transfer maps to your GPS unit. The advantage with this method is that you can selectively transfer only the map tiles you need to your GPS unit (rather than an entire country or region, for example) and you can easily view these maps in the BaseCamp software. I recommend the latter approach.
One final note: if you're looking for international maps (and you're not as multi-lingual as you'd like to be) I highly recommend navigating to these links either with Google Chrome or using Google Translate. These will both allow you to perform instant translation on any page that's not in your native language. Sure, the translations are a bit rough around the edges, but it usually makes it MUCH easier to find the file you are looking for!
- Good quality Colorado map with lots of landmarks. The site also links to a number of other state maps (some by the same author)
- This is a complete set of Italian maps - good quality, but some odd quirks like that the sea is not colored blue (which makes it hard to determine water vs. land when zoomed in).
- A complete set of Norway maps, including different map sets for summer and winter
- A fantastic map of Southern Germany
- A fantastic map of Switzerland. This is really all you need
- NZ Open Source Topographic Map Series for Garmin
- Maps from Land Information New Zealand - downloadable in GeoTIFF and Shape formats. Not immediately usable on a Garmin receiver, but a useful paper map or to be pulled into a tool like OziExplorer
- Shonkymaps - http://shonkylogic.net/shonkymaps/
- Rough mapsource resource that allow one to build GDB files for the GPS. These aren't too bad though they don't allow a good drill down of contours for serious backpacking,
- This site has a great wiki of hundreds of map products all over the globe. Most of the links I have above I found from this site. They also have a lot of great resources for installing the map products if you poke around a bit.
- This site allows you to dynamically create a map for anywhere on the globe using OpenStreetMap source data. I typically use these maps as a "fallback" since they are usually very spartan on formatting - all of the salient map detail is available, but the quality just isn't as high as a map product that a human has taken the time to polish.
- While not free, the Garmin base maps are terrific and they really do deserve to be mentioned here. If you can't track down a good map for the region you're headed to, it may be worth shelling out a little cash to pick up the Garmin map for the region. There new BirdsEye feature also allows you to download image tiles (non routable) of either satellite imagery or native Topo maps (such as USGS Topo quads). The BirdsEye imagery is typically only about $30, which is really very reasonable for high quality map scans.