Keeping track of bibliography references for an entire research group
I accomplish this using the groups feature of Mendeley.  It works on all three major OS's, allows you to share bibliographies easily with both your group and external collaborators.  It also allows something that I think is very important -- lots of bibliographies on particular topics within the realm of what my group does.  See, for example 


http://www.mendeley.com/groups/1045561/runge-kutta-stability-regions/papers/
http://www.mendeley.com/groups/1070421/nonlinear-hyperbolic-pde-solvers-on-gpus/papers/


I should mention that Mendeley's web interface to bibliographies is awful.  But the desktop interface is quite nice and (most importantly) can export Bibtex.

Mendeley does store your data in the cloud (if you consider a bibliography to be "your data").

Update: I stopped using Mendeley when it was bought by Elsevier.  I haven't found a satisfactory replacement.