I REALISED after reading David Allen's Getting Things Done (well, most of it) that Gmail has been designed almost from the ground up to make implementing Allen's personal productivity system, easy. Now I realise this is probably not news to anybody, least of all Gmail aficionados (first time I've ever used that word--whew). But I just feel like going over some of the productivity-enhancing features in Gmail that have been going through my head lately.
First of all, the idea that you don't use folders--you just get an inbox and an archive for all messages, and you can tag messages with any--and multiple--tags that you like. The point of this is that you treat all incoming mail as something to be processed, something that requires your attention. And you view the inbox as a place that holds this mail--mail that you need to process as you soon as you get some time. Any mail you don't need to act on--like information for your reference, you might as well put it in the archive. And in fact, you might as well set up filters in Gmail that automatically archive it, if you know that you always get this kind of mail from somebody (or whatever other criteria). For example, I've set things up so that mail from the online vendor I buy bus passes from automatically gets archived, since I don't need to act on it--it's just order confirmations.
So in this way, you get your inbox down to zero messages, ideally. Everything that you don't need to act on, and everything you have finished acting on, has been archived. Anything you didn't want to keep, deleted. Simple, but it feels good to know that you're on top of your email instead of the other way around.
And of course, we do need to categorise our emails--work, personal, sports, whatever--and here we have tags. So you can tag messages any which way, for maximum flexibility, and then just click on the tag name to see all messages tagged the same way.
Of course, the real power of these categories, or labels, is that you can use them to find, say, all emails from friends about school--if you've tagged them as such: just search for `label:Friends AND label:School'. There's a lot of flexibility. After all, Google is all about the search.
So Gmail is all about these two concepts: the inbox, and the archive. Because broadly speaking, you're always either processing email, or finished with it and just keeping it around for reference. Everything else is icing on the cake.
Shameless plug: I highly recommend Getting Things Done. It's the one self-help book that I've ever read, but I can pretty much tell it's the only one I'll ever need to.