Most handheld Garmin GPS units come with a baseline road map that displays major roads for either the Eastern or Western hemisphere, plus the ability to display greater detail using uploaded map data. Garmin sells a line of CD/DVD disks with data for various parts of the world, and third-party sources offer maps with greater details for specific countries (a Google search should reveal even more). Many of these datasets are expensive, and data is unavailable for some areas of the world. There’s a cheaper option that offers worldwide data, including coverage of areas not covered by commercial products, but with some limitations in quality and detail.
The Wanderlust Worldmap is a community site slowly assembling and improving a worldwide map of roads, points of interests, and contours that can be uploaded to Garmin GPS units using the standard Garmin MapSource software. Full access to all the data requires a 25 GBP registration fee, but you can download a sample dataset for free if you register at the site. Here are some sample screenshots from MapSource, downloaded from the website (click on the images for a larger view):
Looking at the maps in the evaluation download, quality and level of detail vary enormously from one area to another. Some areas show detail rivaling that of standard Garmin maps, while other areas have less detail, and don’t even label roads with a name. Still, you can’t beat the coverage area or the price, and presumably with time the overall quality of the maps will improve as community members create and update new maps. At the very least, the evaluation version is worth a free look.