The coolest features:

"Once you’ve started searching, you’ll notice that tweaks to your query show up in real-time as you type each character, the same way they do on Google Instant. And it shows results for partial-word matches (“Tech” would match for both “Technology” and “TechCrunch”) — which Gmail doesn’t do."

And "a message with 500k of attachments showed up in my search results within a second of receiving it. Not too shabby."

Full disclosure: I work at Greplin :-D I'm just proud of the very cool stuff my coworkers are building!