Maps aren’t avaiable yet for the entire world, but you might consider a GPS with a large memory, to which you can download maps available for countries you may be visiting.
I know that Garmin offers something called nüMaps Lifetime™ that updates your maps, and that you can get maps of Europe, North America, the Middle East, Australia and New Zealand.
TomTom allows you to purchase map downloads over a broadband connection for:
North, South & Central America (North America, Argentina, Brazil, Canada & Alaska, and/or Mexico),
Europe, (Western & Central Europe, Alps plus major roads of Western Europe, Benelux (Belgium, Netherlands & Luxemborg), Central & Eastern Europe, France, Germany, Austria & Switzerland (DE_AT_CH), Germany, Austria & Switzerland (DE_AT_CH), Greece, Iberia (Spain and Portugal), Italy, Poland, Scandinavia (Denmark, Finland, Norway & Sweden), Southern Europe, United Kingdom & Republic of Ireland),
Middle East & Africa (Mauritius, Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates, South Africa, Southern Africa, Turkey
Australia, New Zealand and Asia (Australia, Hong Kong, Malaysia & Singapore, New Zealand)
They don’t seem to have maps of other countries besides these. You can go with either downloaded maps (always there when you need them) or you can go with “cloud maps” that rely on you having cell phone access where you. If you are in a dead zone you don’t get maps.
This site reviews a lot of the GPS units out there, covering more brands than most similar sites.