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</a>. 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 <a href="http://www.papersapp.com">Papers</a>, which manages all these PDFs in a very nice way and searches mathscinet, arXiv and the like for you.