Secure online file storage: recommendations?

Hi to all,

We need to set up a secure space from which users can download and
upload files. Requirements are pretty basic (<100 users, a few Gbytes
of disk space, normal bandwidth).
Setting up our own Linux box with a SSH server may not be a feasible
solution due to lack of room space.
Has anybody utilized secure online file storage providers for
professional use and has one to recommend?

TIA,

L.
.