I've been running the Firefox extension GTDGmail for a few days now. It provides a bunch of convenience functions for using Gmail's labels as a makeshift GTD system. Integration with Google Calendar would be nice...but I'm still waiting for that within Gmail vanilla, so I can't really fault the GTDGmail extension for its absence.
I'm already finding that it's really helping with keeping my inbox clean...if a message doesn't fit into a project, it generally doesn't need keeping.
However, I'm unsure how accessible it is as a first-exposure to GTD tools. The docs are minimal; I found myself relying on experience gained from the GTD TiddlyWiki mods (MonkeyGTD is especially good: http://monkeygtd.tiddlyspot.com/) to understand how to use their labelling system.