I have a Psion series 5, and I use realmaps http://www.realmaps.co.uk I find it great! You can scan in maps, register them and navigate on them. I have a CD of maps of my area, I just use photoshop and select the area I want to use, crop it and upload it. It is great if you have a psion. If you want a psion try ebay.
Cheers.