Hello, We have just returned from a 2 weeks trip to Paris and i would like to share with you this little jewel of an agency that I found by chance.
We used http://www.parisforrent.com to book our flat and it was an extremely positive experience. They are based in California and have a great selection of very central and beautiful flats. Their prices are very competitve and you have the level of service you can expect from an american agency even though they are all French!
There are many special deals on their website and I would really recommend contacting them, as they are extremely helpful. They helped us find excatly what we wanted and made our trip positively memorable!
Have a good trip!