One of the best collaborative tools I've found in a long time for development projects is a Wiki because it is lightweight and free-form enough that everyone will use it. We've had one for a while at DSW and I'm stunned at how useful it is. There are lots of overblown document sharing systems out in the world (recently, I had the displeasure of using Microsoft's SharePoint -- ugh!), but the simple Wikis Just Work.
The one we've used for a while at DSW is Very Quick Wiki from SourceForge, a quick, simple, J2EE based Wiki. It took all of 15 minutes to setup and has run flawlessly for months.
I recently discovered an even easier one to set up -- Instiki. The instructions are so complex I'm going to reproduce them here:
Step 1. Download
Step 2. Run "instiki"
Step 3. Chuckle
"Theres no step three!" (TM)
OK, it's slightly more complex (you have to have Ruby installed), but not much. It stores everything in the filesystem (in a single portable directory if you have to move it, back it up, etc.), includes its own web server, and it Just Works. It is now officially my new favorite ultra-lightweight Wiki. And, it's Ruby based, which makes it even dearer to my heart!