ActiveInbox (free, or $25/year for Plus)
GTD for Gmail. The best Gmail productivity tool I've ever found.
To grok ActiveInbox, watch the video. Here are my favorite features:
Those two features alone are worth the setup for me, but ActiveInbox does a lot more than this, too.
Hat tip to Louie for finding this one.
If you use outlook, Outlook 2010 lets you (at least) duplicate these features.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: