[GianFabien]

In my experience (Australia) you cannot negotiate directly with the potential employer you have no choice but to go through the recruiter.

I have heard of people who got a "signing bonus" at the end of their probation period which was kept a secret from the recruiter.

Personally, I usually asked for a condition in the contract for a review at the end of 6 months. In that, I would negotiate a 10%+ increase. Which is usually easy when they see that you are competent and do good work. In my experience, that approach works in small firms and hardly ever with large firms.