Speaking of useful tools for scientists, one that I really have learnt to like is Papers. Papers is what I would call the iTunes of bibliography management software; It's not the most feature rich of them all but is simple and very intuitive to use and when you have to deal with collecting papers without it you tend to feel a bit lost.

Two of the particularly great features about it are how it handles importing and organising the papers. Importing is either done through dragging & dropping a PDF or a citation file into the application (finds title, authors etc automatically from either) or from various search and import filters (e.g. PubMed, Connotea). It also knows where to download papers in case you import them from, say, PubMed.

When it comes to organising papers, you handle them like songs in iTunes by throwing them in collections and automatic collections (think playlists and automatic playlists in iTunes).

All in all this wonderful (and cheap) application makes reading papers just that little bit easier and more fun!