I had been using the GTD outlook plugin and found this to be helpful in minor ways. It's main fault in my mind is that it takes the GTD model, where the project / task is the center of things and actaully makes email the center of things. It is really a poor system to capture "all items" and there is not an easy way to get the information out and into a trusted system.
Download Squad just pointed out an alternative to the GTD plugin and I am finding it quite interesting and useful. It is not strictly a GTD Outlook plugin but it does seem to add tools that make Outlook a much more powerful system for dealing with and taking advantage of the mountains of email we all get. I am looking forward to using this tool for a few weeks to see if it can help me get and stay organized in my pseudo-GTD life.