Web resources or other resources for villa rentals in Europe? (Spain, France, Italy etc.)

Here are a few just by Googling.

Amalfi Coast 1, Amalfi Coast 2

Here is all Europe: http://www.rentvillas.com/

In Belgium try http://www.belsud.be

For France, a great site for renting farmhouses in coutryside or small villages is Gîtes de France.

I'd take a look at Air Bnb, it's starting to really appeal to me for big bookings, or for non-standard hotel bookings.

Just an addition to Mark Mayo's answer, Roomorama is also a website that similar to AirBnb.

In Spain the biggest (and also the one that offers better prices) is idealista. You can try to find by "rent" "holiday" in any of the country areas. Good luck.

The largest such site is http://homeaway.com, which is a big roll-up of many smaller sites that got bought by the same company.

A great search site is Otalo, it searches all the leading vacation rental sites (like airbnb, HomeAway, Niumba, etc.) for you. They list over 500,000 holiday rentals for just Europe:

Another good website I found was this: http://www.provacances.com/