An anonymizer or an anonymous proxy is a tool that attempts to make activity on the Internet untraceable. It accesses the Internet on the user's behalf, protecting personal information by hiding the source computer's identifying information.

Depends on what your talkin about but usally onbehave

You use on your behalf when you are representing me and in your behalf when you do not want to put a lot of emphasis on what you are saying.

in behalf of is incorrect usage (Delete me if I am worng)

A behalf is an advantage or favour, to do something on someone else's behalf is to do it in order to someway assist them.