Garmin GPS is what most people use, there are many different unit and prices, if you want a low price unit that gives you directions than the 200 series will do the job, If you want MP3 function and lane assist and intercom compatible than you might want to check out the 700 series. Go to garmin.com and you can see all the different units and what they do. Unless you get a motorcycle specific unit it will not be waterproof, but just put a baggie over it or take it off when it rains. I have a Garmin 200W and it is an older unit but it does everything I need it to do,. You can also finds some good deals on GPS's on e-bay, a recondition unit comes with a new unit warranty and save you some money.
There ae many models of very useable GPS units out there. do your homework first and make a list of what you want it to do, and then check the models that do that and go shopping for both availability and price. I'm pretty Garmin biased so I won't offer too much other than the Zumo and Nuvi are very popular.
I recently purchased the Garmin Nuvi 500 and have used it several times with good results. Its a portable unit that is waterproof. I have used in the car and on the bike. Cost was $199. Have used the mapping program to put together a route for a three day 1000 mile ride for a group and it was flawless. Used it to identify gas stations, hotels and restaraunts as well. Would definately recomment.
A few have posted the same GPS question including myself. Garmin seems to be the brand to get based on all the replies/reviews. I currently have a TomTom 730. Screen washes out in the sun when it's on the bike. Works fine in my car/truck. Have not tried putting a hood on it and it's not made for rain etc. Also TomTom support is not the greatest in my experience.
I am leaning towards the Garmin 660 or 665 but MAN are they expensive! Searching the Net for a refurbed one now. Maybe I'll pop for a new one around Xmas if I can't find a refurbed one.