Benchmark datasets for collaborative filtering
The obvious answer would be the Netflix prize dataset, there is a lot of research into it and most CF algorithms have known scores in it. 

There are other available datasets that are usually used as benchmarks:


Movie lens Dataset: a 20 million ratings dataset used for benchmarking CF algorithms;
Jester Dataset: a joke recommendation dataset with more than 6 million ratings;
You can find many more datasets in this link