Open Applications are Yahoo!'s answer to Google's iGoogle widgets and OpenSocial platform.   Yahoo Open Apps are part of the Yahoo! Open Strategy (or Y!OS for short) - Yahoo!'s push to "deliver open, industry-leading platforms that attract the most publishers and developers."

Positioned as "Social Platform for Yahoo", Y!OS really represents a set of open API (Flickr, Yahoo! Mail, Yahoo! Maps, Yahoo! Search/BOSS, ie) along with the "new" Yahoo! Application Platform (YAP) that allows developers to build applications that plug directly into Yahoo! (social) pages (MyYahoo, Yahoo 360, i.e.) and presumably other Yahoo! properties (Flickr, MyBlogLog, i.e.).

Although the language of what Yahoo! is offering make be confusing - Yahoo! Social Platform (YSP), Yahoo! Application Platform (YAP), Yahoo! Open Strategy (Y!OS) and a whole bunch of other three letter mnemonics that start with Y - the idea is not! Yahoo! wants to compete with Google and Facebook to be the start page for the Internet and needs social media and networking application that captivate a Internet-savvy audience to pull it off! It has the popular email client, a bunch of popular media services (Flickr), good IM presence, it has most of the basic API's, and all it needs is the glue. Can Yahoo! do it? Perhaps.  Here is my initial feedback:

• The strategy makes sense - why site around waiting for Facebook, Google (perhaps MySpace) to take over the world - fight back!
• Yahoo! has all the basic API's you need to deliver a compelling social platform.  The same can be said for Google, but not for Facebook or MySpace.
• Yahoo!'s approach to user interface design, SDK and architecture design and platform support has generally been better than those of its competitors. Yahoo! Mail is a lot nicer/cleaner/intuitive than Gmail.  Yahoo! was first with a Flash-based Map API that still looks a lot more polished. From a UI perspective, Yahoo! kicks butt!
• Yahoo! vision appears more comprehensive (think 360) and well thought out than some of the others. Google seems to have a lot of small disparate APIs but nothing to really pull them together to deliver real power. OpenSocial (which appears to be the only available glue) will probably never be Google-centric enough (given it's an "open" standard) to pull this off, without becoming even more complicated than it currently its!
• Yahoo!'s once owned the Internet's start page. The MyYahoo! personalized portal used to be the starting point for most user's online experience in the late 90's when I was busy starting Epicentric - the enterprise portal builder, before the dawn of social networks like MySpace and Facebook.
• Yahoo!'s traffic and network cannot be underestimated. Yahoo! is ranked #2 in traffic by Alexa (Google is #1) and Yahoo Mail has $275M global users. If Yahoo! pulls this off you can expect some powerful social applications that live and interact in such Yahoo properties like Yahoo TV, Yahoo Music, Yahoo IM, Yahoo Toolbar, Yahoo 360%, Flickr and the list goes on!
• Yahoo! has some innovative services in the works including Yahoo TV Widgets and are currently scouting out some some rather interesting acquisition opportunities, and is playing around with third-party APIs and technologies like Adobe AIR for new product/service extensions and concepts.

As far as the platform is concerned: Applications support both a small and canvas yiew. Similar to Facebook, YML - Yahoo Markup Language Platform gives developers access to social data and features. Its Caja JavaScript sandbox eliminates threats posed by rogue Javascript code. It comes with a presence API and supports OAuth. The YAP SDK has great support for PHP and did I mention that it YAP supports OpenSocial 0.8 - pretty sweet! The platform comes with a web-based development environment (that's nice) and apps will off course be featured/listed in the Yahoo App Gallery!

More to come on this subject later on this week as we embark on a handful of Yahoo Open Application development projects!