I have a rules wiki (hosted on wikidot) and a forum for campaign and rules discussion. I use Google Documents for campaign notes, Excel to keep track of resources I'm using, and Word for documents that require a more complex layout (handouts, my creature compendium, and such).
I once got very close to creating a D&D rules database; I completed the relational database model design and started working on the interface, when I got side-tracked into other projects. Once I'm done with my extensive d20 rules revision (which may take another year or two), I may go back to that project. My ultimate desire is to have an easy to reference rules database, character generator, and creature builder. Yeah, something like the DDI, but for my own ruleset. Platform of choice is C# (VS.NET)/MS SQL Server (the Express Edition is free and works great).