A few useful online document management sites which I personally find very useful
1. Box.net (www.box.net)
This is a cool online utility with a lot of features that not only allow you to upload and share a lot of documents but also is integrated with gmail, twitter, linkedin etc., also allows you to convert the files to pdf versions, allows to send fax and convert the text to speech if you have ispeech account.
2. Scribd (www.scribd.com)
Scribd is one more famous utility of which most of us already know. Scribd allows sharing of documents online.
3. Google Docs (http://docs.google.com)
I would consider google doc as the pioneer at least for me for it was google docs with which i started sharing documents in the beginning. Google docs did some active changes in the beginning but somehow has lost its luster now..
4. Zoho Docs (http://writer.zoho.com)
Zoho writer one more in the crowd that offers online document sharing and collaborations. Ideal for a team working in a single document.
5. Windows live workspaces (http://workspace.office.live.com/)
Last but not the least windows live certainly emerges as a winner with lots of new features a huge storage space (25G) and a very intuitive document sharing with a variety of documents in its clout.
I almost use all of the above .. I would surely believe there are lot more out there which others use do mention those in the comments section if you think it could be useful for others who are unaware of it till now.