Open Source Maps
Here's an OSM page with Great Western Park already updated
I've been cutting my own OSM maps from openstreetmap.nl. Works very well, with usable maps of Namibia and Egypt, and good selection of POI. Maps also routable, so good for Sat Nav if driving. I've not used them too much in the UK, because I've got the full OS 1:50,000 maps, and the OSM maps don't seem so good at footpaths and walking.
Interesting curiosity also is that the OSM Egypt maps online show railways, and power lines (eg here from the Aswam High Dam), but these (layers?) don't get included in the maps generated for the GPS.
This wiki seems to list all available sources of maps cut from the raw OSM data.