eXpresso, a year-old venture-backed California start-up, has taken Excel to the clouds with Microsoft’s blessings. PowerPoint, Word and PDF files are set to follow by the end of the summer.

eXpresso webifies legal Excel users and lets them share, edit, download and print their spreadsheets over the Internet; the number of people collaborating has virtually no bounds.

They send their spreadsheets to eXpresso’s cloud (a k a a secure server) – an SSL encrypted process, eXpresso CEO George Langan says – where the password-protected data is stored redundantly in an Oracle database, accessible via an IE 6 or later browser. And any changes made on- or offline (the later complements of an add-on) are automatically synched up.

Access also bridges other clouds such a Salesforce.com and Cisco/WebEx Connect and possibly, in the future, IBM’s Lotus-based Blue House, now in private beta, Google’s “My Stuff” online storage and Microsoft.

Imagine then Google Docs users or Salesforce.com users creating spreadsheets in eXpresso on their platform that appear on their platform as functioning documents of that application. A Google Docs user would see a Google Docs spreadsheet and a Salesforce user a Salesforce.com spreadsheet.

The fact that eXpresso is reaching out to licensed Excel users explains why Microsoft has inducted the company into its picky new Startup Accelerator Program, which helps promising early-stage ventures that add value to the Microsoft ecosystem. The criteria Microsoft uses include marketability, growth potential, funding, management and strategic potential.

And the fact that eXpresso rigged its patent-pending infrastructure widgetry out of Java and Oracle pretty much insures that it’ll never be a candidate for acquisition by Microsoft, George said.

eXpresso, which is governed by an invitation process and a system of rights and privileges, has been in beta since last August and recently went live.

There’s a basic version of its product available for free and a full-blown ad-free version called eXpresso Pro now available for $15 per user a month or $79 per user a year, with volume rates starting at five users of the paid Pro.

Pro adds cell tracking and compliance-based audits, rollbacks, real-time archived chats about the work at hand, editable cell regions and e-mail change alerts.

George says that after the beta eXpresso’s got 300 paying customers and another 700 users availing themselves of the freebie.

eXpresso consists of 10 people in the US, 14 in Vietnam and a few Romanian contractors. It’s backed by Novus Ventures, Rocket Ventures and IVF Ventures. Its first round was worth $2 million and the $3 million-$5 million second round that it’s currently out for should take it to profitability by this time next year, George thinks.

Available for a free 30-day trial, eXpresso is compatible with Office 2000, XP, 3003 and 2007 and supports .xls files. The free version is limited to five spreadsheets, a cap that disappears with Pro.

Ventana Research puts the global market of business application users at 166 million. Buoyed along by a consensus prediction that two-thirds of them will adopt cloud computing applications over the next five years, George says they’re all potential subscribers.