MSN combined with Microsoft Communicator works well, linked in discussion groups and yousend.com for moving files around.

www.cubetree.com that is now owned by Success Factors. Is excellent.

Box.net all the way. Great document collaboration and sense that is still the standard for creating materials it works great. I have tried most document cloud solutions and I like box.net the best with dropbox as a close second.
Box has a great interface and more importantly excellent integrations with other apps. It also has many ways to share files with clients as well as other people in your workplace which is great.
Just my two cents. Next up I would put google docs. Google forms is amazing.

we use www.box.net also in conjunction with www.cubetree.com. They do different things but both are excellent tools.

We've been using HyperOffice for several years for document storage, shared calendars, contact and project management. It's been the perfect solution for our virtual workplace. I'm always keeping my eye out for other cloud technologies, but I haven't found one yet that can offer the same suite of solutions all in one place. www.hyperoffice.com

We also use box.net for larger files when we don't want to store them in Hyperoffice. We love its ease of use and the file share links are great. zShare would be a second option.

We prefer www.incipi.com powered by www.openatrium.com but we are slightly biased.

HyperOffice, Google, Zoho and LotusLive are all excellent on demand collaboration suites. Below are some free resources to help evaluate them and others:

•Whatis an Online Collaboration Suite, and Why Should You Care:

http://www.smb-gr.com/smb-education/what-is-an-online-collaboration-suite-and-w
hy-should-you-care/

•Blogseries: Are You Ready to Move Beyond Email?

http://lauriemccabe.wordpress.com/2010/09/24/are-you-ready-to-move-beyond-email-assessing-online-collaborati
on-suites-for-smbs/

•ComplimentaryResearch Study: Moving Beyond Email: The Era of SMB Online Collaboration Suites:

http://www.smb-gr.com/contact/moving-beyond-email-the-era-of-online-collaborati
on-suites/

We use DeskAway for work tracking and collaboration.

We've tried out around 7 or 8 different platforms and have settled for a new product called Groupcamp.It has all the apps we needed: files, tasks, time tracking, discussions and a WIKI which is to be released soon.We're really thrilled. There's a billing exemption period up to the end of this year.