Europe has many of the same car rental companies you'd find in the US, such as Hertz, so if you feel safe with an internationally-known company you have that option. Personally, I'd recommed
Auto Europe. It's a trusted name and seems to have good rates. I got a quick quote for 21 days in Barcelona and the going rate for a normal car was a little over $30 a day. Of course, I said I'd be picking up and dropping off at the same spot, which might help.
I'm assuming you won't be, but remember the heavy cost of picking up a car rental in one country and dropping off in another. It's not like doing it between states in the US, the fees could astound you. Good luck!