Starting when I moved to New Orleans and began working with Jedidiah Horne on disaster recovery research, I've been trying out the latest in free online collaboration tools. Here are links to some that I like:
Google Documents - This is a google product which can replace Microsoft Word and Excel. It is perfect for storing notes and abstracts and allows two or more parties to edit at the same time. Also, it allows me to easily share some of my notes easily with other researchers as needed. Google documents currently does not support most advanced foramtting that word does, but it is great as far as collaborating on content. As far as spreadsheets, you can't write macros or use some of the most advanced features that Excel offers, but you can easily edit and share basic spreadsheets with anyone. There is a bit of a lag when using it as it recalculates cells, but we have found it highly usable and much better than emailing spreadsheet versions back and forth.
Gliffy - This is a non-Google product which replaces Microsoft Visio. I've used it to collaborate on a few flow charts to explain the funding streams and the decision making processes. Gliffy won't allow two people to edit the same page simultaneously, but it does have an interesting feature which allows you to post an image of your chart on a website which will be automatically updated each time the chart is updated.
Google Calendar - Google calendar is great for sharing a calendar and allowing multiple people to both update and access it from anywhere. I guess this replaces Microsoft Outlook's calendar feature. Its also a good way to easily add a calendar to a website with an easy to update backend.
RememberTheMilk.com - This is a great way to share and collaborate on a to-do list. It has a great ajax interface and is very easy to use. They've implemented numerous ways to keep it updated too, including offline with google gears and via sms through twitter.
del.icio.us - I use this service for saving and tagging links, and sharing them with others. Every time I find something I think I might need later I tag it. Check out the del.icio.us firefox extension to make it even more useful.