The best ones are in Spanish used by normal Spanish people.

These are the 3 best websites:

Granadainmo:
press the venta button and then use the search system
http://granadainmo.com

Fotocasa
http://fotocasa.es

Compraventa
http://www.compraventa.com


There are plenty of English websites dedicated to real estate.
Most of the English websites concentrate on property in the countryside rather than in the city centre.
One that is well organised is this one
http://www.property-net-granada.com

BE CAREFUL
If you are buying property in another country you need to have proper legal advice
and you shoudn't rush into anything unless you know what you are doing.
If anything seems too good to be true it probably is.