Before going to the reference page read some interesting remarks from professionals:

"20 Rules for Dealing with Agencies and Direct Clients"

rule12.

Regarding references: never, under any circumstances, give out references. Giving out 2 or 3 references is common practice when applying for a permanent position, but as freelancers we cannot do that: we are legally and ethically bound to keep any and all information regarding our clients confidential. Therefore, suggest to the agency that they could send you either a 200-word test or a small job for which they would have to pay you a minimum fee (“the proof of the pudding is in the eating”). This way, the agency does not take on too much risk and you would not have to breach your clients' confidentiality. Remember: when you see a new doctor, you cannot ask the doctor for his/her patient list either!!!

However, a few customers, specifically gave permission to use certain projects as reference. Which you can see here.