The two sites we found the most helpful (and these are literally two of many, many sites) were Propriétés Le Figaro and Green Acres. These two sites made it the most easy to search for homes and gave daily e-mails with new homes that popped up that were within our requirements. We did, however, stumble upon Maison à Vaison on Rightmove.co.uk who acts as an English liaison between the French agencies and the English-speaking buyers. 
We've also talked to more people that I'd ever expect who purchased their home on Leboncoin.fr
If you aren't familiar with the site, it is our equivalent of Craigslist.com here in the states. It is actually where I found my room for rent when I moved to France to teach English. It has EVERYTHING and is just a good over all resource. This site would best work for people who are actually in France and not shopping from abroad, as these homes are more likely than not sold by the actual seller.
When you have your wish list narrowed down and really know what you want, then the next step is to start browsing using your wants and see what pops up. 
When we started this process, I found it really helpful to start a google doc of the homes I liked, the link to the home, the agency listing it and their contact and the region/town it was listed in. That way, we were able to keep things straight. 
Once you start browsing, you will quickly start to gravitate towards certain homes, making it easier to narrow down your search going forward.
HELPFUL TIP: Don't get discouraged by the photos. MANY, MANY homes only had 2 or 3 blurry, nondescript photos. We were told it is so that other agents couldn't tell exactly which home was for sale and then approach the seller and poach the listing. I talk about this a little in my blog, but homes can have an infinite amount of agents/companies listing it. This sometimes makes it confusion to try to book a viewing. 

Part 2 Continued….