Recently my partners and I spent some time discussing three of our recent investments – Spanning, Yesware, and Attachments – which are each applications built on top of Google Apps. Specifically, they are built for Google Apps and available in the Google Apps Marketplace or the Chrome Web Store.

Each company is going after something very different. Spanning is all about cloud backup. Attachments is all about getting control of your email attachments. And Yesware is “email for salespeople.” However, they have one very significant thing in common – they are all deeply integrated into Google Apps. In our thematic definition, they are in the Protocol theme.

The Google Apps ecosystem snuck up on us. We have all been hardcore Google Apps users for the past year and are psyched and amazed about all the easy integration points – both into the browser and the various Google Apps. In the past, we would have been more focused on “email as a datastore”, which would have resulted in multiple platforms, including of course Outlook / Exchange and IMAP. However, the pace of iteration on top of Google Apps, and the ease of integration is spectacular when compared to other platforms.

Notably, when the choice of building for Outlook vs. Google Apps comes up, many people who I know comes down strongly on the side of building for Google Apps. Their mindshare for cloud based business apps far outpaces Microsoft. A decade ago, Microsoft made a huge push with Visual Basic for Applications and the idea of “Office as a Platform” and – while plenty of interesting tech was built, something happened along the way and the notion of Office as a Platform lost a lot of visibility.

Theoretically Microsoft’s huge installed base of Outlook / Exchange users should drive real ISV integration interest, but the friction associated with working with Microsoft seems to mute the benefit. And – if you’ve ever built and tried to deploy an enterprise wide (say – 100,000, or even 1,000 seat) Outlook plug-in – well, I feel your pain. It’s possible that with Office 365, Microsoft will re-energize focus on Office as a Platform, but I haven’t seen much yet.

While Google has been building this all very quietly, I’m extremely impressed with what they’ve done. Companies like Yesware are able to release a new version of their app to all users on a weekly basis. For an early stage company that is deep in iterating on product features with its customers, this is a huge advantage. And it massively simplifies the technology complexity to chose one platform, focus all your energy on it, and then roll out other platforms after you’ve figured out the core of your product.

I expect to see versions of each of these products expand to work with Microsoft – and other – ecosystems. But for now, the companies are all doubled-down on Google Apps. And I find that very interesting.