GTDGmail
I'm a big Gmail fan. I switched to it quite long ago (it was still in early beta - I have a friend who works at Google). The moment I tried it I dumped Eudora and Outlook (except for work where Outlook is all but unavoidable) and moved all my personal email to Gmail. The fact that it is only available online seemed a small price to pay for the benefits (and now with Google Desktop Search you can sort-of read your email while offline).
Now GTDGmail has come along as an add-in to Gmail. It is far from the ultimate GTD plug-in. In particular, it is missing one key feature for me, the ability to defer an action until a specified date and time and then have it magically reappear in my inbox. Perhaps other people can remember to look at their Next Actions all the time but, at least for us ADDers, this is definitely a near-impossibil
But besides this, it is actually a pretty good tool. It does a beautiful job of synergizing (not a word but should be) with one of Gmail's greatest features, labels. The different states of GTD (eg: Actions, Finished) as well as the different contexts (eg: @Home, @Work) all become labels. There is some minimal w orkflow built into the system to help you manage the relabeling. For example, when you mark something as Finished GTDGmail can be configured to remove the Action or Next Action label. It is still pretty minimal but quite useful.
One use I have of it is, via Gmail filters, to move all of my subscription emails to their own project, archive them (eg: skip the inbox), and mark them as Actions. Then, when I want to see email that I haven't read from a list, I just click on the GTDGmail provided Action link for a specific list's project and it shows me all of the emails. When I read an email and am done with it, I remove the Action label (GTDGmail provides a handy "X" next to each label an email item has that is about a billion times faster than using the "More Actions" menu) or by adding the Finished label (GTDGmail also provides a list of label links that you can just click on to add - once again about a billion times faster than the "More Actions" menu). Then, the next time I go back to the project's Actions, I have unread mail that I haven't seen and read mail that I've seen but want to come back to. If I read an email that seems important but requires more time than I have to give it (the GTD 2-minute rule) I mark it as a Next Action so I'll get to it shortly.
All this allows me to maintain my, much-desired, empty inbox. The only things that go to my inbox are emails that I don't have filters set up for (and Spam that Gmail's filter misses, of course). I can dispatch those quickly. If I ever get too far behind on one of my email lists, I simply mark the older, unread posts as Finished and just never get to them.
Even if you are not a GTD fan, GTDGmail might be worth using just for the added labeling functionalities
One drawback that I have to mention is that GTDGmail is a bit on the slow side. Whenever you click a link that causes the screen to redraw, GTDGmail adds a second or two to the redraw. Worse, the screen redraws without the GTDGmail controls and then a second or two later adds them in. This frequently causes me to click on a link just before the GTDGmail redraw occurs and to end up clicking on a different link because my link just moved. A bit of a hassle (and a deal breaker for some according to comments on the plug-in page), but worth it for me.