I thought I'd kick off this with a discussion about a freeform database.
At Freeform Games, we already use an online database when we're writing our games. We use the database as a glorified plot matrix - and we've used the free option at zoho.com to set one up.
I've written more about that here: http://www.flar.demon.co.uk/freeform/article-plot-database.html
The zoho database is only really good for checking structure - it doesn't do the whole writing as plots and then exporting as characters thing that I'd be looking for in a database.