A recent update to iGTD brought some significant enhancements, one of which is very, very rich integration with Scott Morrison's MailTags 2.0. With v1.4.3, iGTD can process most of the MailTags 2 information attached to an Apple Mail message. For example: keywords in MailTags prefixed with @ will get translated into contexts, while your other tags will simply become regular tags in iGTD. Same with projects, deadlines, priorities and even notes added to Apple Mail messages with MailTags: they'll all effortlessly transfer over into iGTD when importing messages with an F-key.
If this is all getting your productivity and GTD gears turning but you haven't purchased MailTags yet, Scott Morrison and Bartlomiej Bargiel (iGTD's developer) have just offered a partner promotion that should do the trick: for a limited time, when users donate to iGTD, they can contact the author and receive a $5 coupon off the $29.95 retail price of MailTags 2.0. I say 'limited time' because, as of this writing, they have 35 coupons left, though Scott said he's definitely willing to whip up more coupons depending on how much interest spawns from this promotion (I know, I know: I warned them about the TUAW effect, so we'll see).
As an extremely happy and paid user of MailTags and a budding GTD user for whom iGTD finally made sense, this sounds like a great deal for anyone looking to get more productive with their email. MailTags and iGTD: two great tastes that taste great together.