A recurrent Helpline theme is GPS navigation devices for people traveling to Europe. Mainstream options, especially for people renting a car, are:
Renting a GPS device from the rental agency: it is tempting when booking a car, but it is often the worst option. Renting a GPS unit will set you back some $ 15/day on average, car rental agencies rarely bother upgrading their units' maps and you might have to learn on spot how to operate a new gadget that might be setup on a language you don't understand. Avoid it.
Buying a new GPS device in Europe: for longer trips, it might be a worthy option if you don't have an own device. Make sure the unit you are buying has maps for all countries you will be travelling through (all mainstream brands have). It will operate smoothly and more expensive units come with 6 months/1 year of real-time traffic information plans that use their own radio system.
Uploading European maps to the GPS unit you already own: all but the oldest GPS units of major brands (Garmin, TomTom etc) allow the user to buy stand-alone maps and upload them into the device (via USB, memory cards etc). It is a good option, for it allows you to upload maps at the comfort of your home, and then allow you to use a unit you are already familiar with from using it on your day-to-day driving.
Installing an OFF-LINE GPS navigation app in your smartphone or tablet: most smartphones have GPS capabilities, meaning they can receive GPS signals which are free for anyone having a proper device, courtesy of US military. Most people also have simple free apps (like Google Maps) that rely on cellular data traffic to upload maps. This is a no-go when roaming, as loading maps use tons of data and you don't wanna pay for that. A very good solution is to purchase and install a paid off-line GPS navigation app from Garmin, TomTom or other developers. These apps are huge (TomTom Western Europe takes 2GB for installation) but, once installed, they will operate as a stand-alone GPS unit, not requiring any data traffic to operate (they will work even if you disable all cellular traffic, as you should). It is a good option if you don't want to carry an extra GPS gadget.