I am planning on a road-trip from Berlin to Barcelona. I am most interested in knowing if I can hire a car in Berlin and drop it off in Barcelona.
(also, if anyone has tips on routes to take that is also appreciated)
Any large international car rental firm that operates in both Germany and Spain should work. I just searched via the regular homepages of Hertz and Europcar, and both offered similar rates. It won't be cheap though - around 1000 EUR if you allow only a week for the trip, 2000 if you take a month. Be sure to buy a large enough km package in advance.
I tried to do something like this once, but starting in Spain. I found that it was much more cost effective to buy a car even with insurance. If you have a friend living in Europe I would suggest buying the car in their name to make life easier but it is possible to buy a car as a non-resident in Spain.
I've just done a quick test (picking up in Perpignan and dropping off in Strasbourg or in Berlin) and for the same time period it's around 1000 EUR cheaper to drop off in Strasbourg. You might find it much more cost effective to pick up each car in a country, drive it as close to the border as you can, drop it off, take a train over the border, and then pick up a new car. Karlsruhe to Strasbourg and Perpignan to Figueres are both quick and easy by train, or you may even be able to get rates if a company has a "foreign" car you can drive back to the country its from (although obviously this is very dependent on whether they have a car that needs moving). Depending on your route you could even visit Italy and Switzerland with the French car and then Belgium and the Netherlands with the German one without going too far out of your way.
Another option is to drive a car for somebody who wants to move it from A to B - I can't remember the website and a quick Google search didn't bring it up, but there was an article about this in something I read a while ago. I'll look some more and if I find anything I'll post it.