An amicus curiae is a 'friend of the court'. The concept is related to, although possibly distinct from, the 'intervenor'. I think that an 'intervenor' argues for a position, and participates in the proceedings as a form of advocacy. The amicus curiae has a role that is perhaps more neutral, informing the court basaed upon expertise.
A recent development of some interest is the evolving practice of the UN High Commissioner for Human Rights in this area. Over the last few years, she has intervened in four cases, and more such initiatives can be expected in the future. The first intervention, I think, was before the Special Court for Sierra Leone in a matter of direct concern to her, namely, the compellability of a former official of the Office of the High Commissioner. Subsequently, she has intervened as amicus before the International Criminal Court, providing expertise on whether or not investigations could be conducted in Darfur. Her views were somewhat at odds with those of the Prosecutor, and he reacted to them somewhat negatively. She has intervened before the United States Supreme Court in a case concerning extraterritorial application of human rights norms, and before the Iraqi Tribunal in opposition to use of the death penalty.
The practice of intervention before the European Court of Human Rights is well established. A couple of zears ago, the Irish Centre for Human Rights submitted an amicus brief to the Interamerican Court of Human Rights. The judgment referred to our brief.
The various international criminal tribunals regularly receive amicus briefs. Most of them cross the line into advocacy, in my opinion.
I am currently in Geneva and checked with the human rights treaty bodies. They, apparently, have never authorised amicus briefs with respect to human rights petitions, although some have been submitted in the past by human rights NGOs.
This is a great subject for an article, and perhaps even a thesis.