Xobni, the San Francisco company we raved about last year that offers a way to better organize your email from Outlook, is launching publicly with its test version.

Its key feature is a sidebar in your inbox that shows you profiles of the people you’re corresponding with. It makes information easily accessible, such as their phone numbers, past correspondence and files you’ve exchanged with them. We mentioned last week its work to join Microsoft and Yahoo’s email offerings.

It’s a download, which is its main obstacle for adoption, because people are lazy. But Xobni thinks it’s useful enough to overcome that.

Founder Matt Brezina said 140,000 people have signed up for a test account, but he’s let only 50,000 use it so far. Now the beta is open for everyone.

The company has spent a lot of time debugging the service and making it lighter. There was a point when I was forced to uninstall the service because it slowed my Outlook too much. It’s now a lot faster.

Xobni is using viral marketing to spread the service. When you download it, and it starts indexing your email, it will notify you of interesting factoids, such as who you most frequently correspond with, who responds to you most quickly, and who within a particular domain name responds to you most quickly. It then gives you a way to email those people, to invite them to download Xobni too.

Notably, Xobni is also working with third party developers to let them build applications with Xobni. By allowing third party integration for its APIs, Xobni becomes a trojan horse for those third parties to access Outlook integration through a plugin. Microsoft Outlook doesn’t offer a friendly set of APIs for people to plug into Outlook, and so Xobni hopes to become the place developers come to for such access. Salesforce is a good example. If you’re emailing someone, Xobni could show you — through an integration with Salesforce — how many sales calls you’ve made to the person, and how many dollars in computer sales you’ve made to them. Xobni will announce such partnerships over the coming weeks.

Update: Folks in comments are asking for more details about how this integration works. Here’s more from Xobni’s Brezina:

We combine multiple Outlook APIs and use each API for what it is best at. Also, our UI doesn’t use any Outlook APIs but instead uses Win32 hacks to insert the Xobni sidebar in the outlook window.

We’ve done things with Outlook that were never intended by the original designers of the program or the API. The sidebar UI is one piece. The other piece is our data store. Outlook’s data stores are too slow, no matter which API you use. So, we keep our own store of the data on the user’s computer that is several orders of magnitude faster than the fastest Outlook API.

75% of our code base exists outside of the Outlook API integration.

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