You're right, now probably isn't the best time -- if I had booked all of my weeks and didn't need to do it, I'd wait until January 1 to start my free trial periods.Is now really the best time to go to sites offering three month trials or is there a better period to try them in.
holidaylets.net has a free trial, and although it didn't work well for me, it is a successful site and many people have been happy with it.
holiday-rentals.com (same as .co.uk) has been very successful for me, as has VRBO (and French Connections, but you're already on that, right?). Neither have free trials that I know of, so it could be worth it to start on those now if you still have unbooked weeks.
Everyone here will tell you something slightly different, though, so if you're looking for us all to agree on the best sites, that's unlikely. The best thing to do is follow the advice on viewtopic.php?t=257 and take our reviews into account once you have that list of potential rental sites.
Cheers, and congrats on booking those last-minute weeks!