ITunes for Research Papers
mendeley.com
mendeley.com
I doubt these prices face much market pressure since universities generally purchase institution-wide access; papers in those databases can be downloaded freely by anyone on the university network. I'll bet that people like me, not currently affiliated with any school, make up a tiny fraction of their market.
Pubmed and later Google Scholar provided the first great leap in allowing article citations to be found quickly from a centralized database. But generally you had to grab this citation and then trudge through the journal's article search interface via your institution's proxy to then retrieve the actual paper. Being able to click Buy Now, or Access Now, and get the actual paper via a flexible authentication system that works across institutions, would be the next revolution. I don't see any indicator that journals are moving to build this, though, so we are indefinitely stuck with an endless patchwork of incompatible e-commerce sites and institutional proxies.
The result is that a bucketload of undergrads are still stuck collecting PDF's via the gateways as their daily drudgework, allowing primary investigators in universities around the country to put off finding actual labwork for the undergrads to do ;-)
London is coming. :)
edit: Missed the "free" part. Papers $40.
I might not be up to date on all the features of Papers but I am pretty sure they never added a better way to sync between computers.
In the last release Papers did add the possibility for citation export to Word, however managing the styling for different journals was far from ideal. I hope Mendeley will solve this for me.
On the other hand signing up created a public social network profile, subscribed me to a newsletter and updates, and seems to offer no way to delete the account. Great, I just wanted to play with the software.
And the official browser support page: http://www.zotero.org/support/kb/browser_compatibility
EDIT: I just realized it says "also available for". Ignore my ignorance :)