Windows/Mac/Linux: If you've ever needed to use maps from popular online map services such as Google Maps and OpenStreetMap, GMapCatcher browses and downloads online map services for offline use.
Using regular search queries such as City or State, GMapCatcher browses online maps, storing the map "tiles" as you browse. When you're offline you can open GMapCatcher and access the previous maps, interacting with them as though you were online — the catch of course is that without internet access you won't be able to load sections of the map you haven't previously browsed to.
GMapCatcher is freeware and available for Windows, Mac and Linux. Have a favourite tool for offline browsing, map-related or otherwise? Let's hear about it in the comments.