Check it out at http://www.activeinboxhq.com
In short, it creates a better Gmail to help you: 1) get your inbox empty, 2) ensure everything gets completed, 3) do things faster in Gmail.
At its heart, it transforms emails into a flow of actions. It uses GTD principles in Gmail for great control, and adds neat efficiency shortcuts like Previous Emails to write better replies.
This is the public beta. We have a long history... but really briefly, a hobby turned into a full-time uISV about a year ago when we received $20,000 in donations - and realized the support it had! And the potential to do something really interesting to bring emailer's peace of mind. If I can share any extra info that would be of interest, please just ask. (I don't know if that's a good or bad number for donations, but I'm really mindful that we're still in a very humble place).
Would love your fresh eyes to give that great and honest HN feedback on the website, ActiveInbox and anything else that grabs your attention :)
1) instantly see emails when you reply to catch up on recent discussions 2) label-emails-as-you-send-them, so you can add tracking labels to the emails you send 3) right click any contact/label to see recent conversations
... and a few more besides, all customizable :)
For anyone worried about security - as I would be - it does not require any Gmail username/password as it works within the browser; and all XHR requests are there to see (and the source is openable, although it's minified).