I recently downloaded Colwiz, which is a collaboration tool combined with a paper management system. The homepage is at <ahref="http://www.colwiz.com">http://www.colwiz.com. It is far too early for me to write a review about this, but I may come back to that.
Another gem I use a lot is Papers, which manages all these PDFs in a very nice way and searches mathscinet, arXiv and the like for you.