Apartments / Flats in Valencia, Spain
To find a private apartment landlord or a shared flat long-term, look at www.segundamano.es or valencia.loquo.com. These are large general resources of private ads, including many apartment / flat ads. Some ads will be short-term, especially during events.
A more convenient, younger, partly student-oriented www.easypiso.com also does a very good job in Valencia, especially for erasmus students. You do have to pay to get the full abilities there though, the free version is quite hampering.
There are also a few short-term apartment rental agencies in Valencia, to suit your holiday or event needs.
The most reputable and comprehensive of these is definitely Accommodation-Valencia.com. They have a good range of apartments all over Valencia and high standards of service. The company has a whole separate subsidiary on the America's Cup 2007 apartments.

Aparturist has a 16-apartment block located very centrally.
Apartaments del Carmen will settle you in a couple of flats they
have in the central bohemian district.