Would you keep a daily diary on Twitter? Or use it to decide what to wear? Or use it to bypass text-messages charges in international countries?

The possibilities for Twitter are about to get a lot richer with the ability to add annotations to any tweet. A handful of developers got the opportunity to play with the new application programming interface over the weekend at a company hackfest in San Francisco.

Annotations, which launched in April at Twitter’s first developer conference and will go live sometime later this year, are a way of marking up tweets with additional data. Instead of having a simple 140-character tweet, you could make a note that the tweet is about the weather or a movie. Other apps will be able to interpret this and display or interpret the tweet in a special way. It has the potential to make the experience of the microblogging network feel a lot more media rich and powerful. (We suggested a few possibilities for “Annotations” here based on conversations with different Twitter developers.)

A dozen or so developers showed off a few works in progress at the company’s headquarters yesterday. (Note: These aren’t finished products, just ideas people have hacked together in a few days).

Fab or Drab, developed by engineers at San Francisco-based Crowdflower, helps people decide what to wear through an iPhone app. You take and upload pictures that friends can vote “Fab” or “Drab” on. (In typical fashion, Crowdflower, which specializes in farming out microtasks that computers can’t solve to thousands of people, says it can also pay strangers tiny amounts of money to judge your photos too.) Another app, Tazpic, brings some classic Facebook functionality to Twitter with photo tagging. 5Slices lets people keep a daily journal of their lives through Twitter by recording five words a day that describe how they’re feeling. Another developer built an Android to SMS gateway that basically lets people text each other internationally without incurring expensive charges. You can see all of the projects here.

Twitter also revealed a few more details about “Annotations.” The biggest foreseeable problem with them was whether different developers would agree on a taxonomy or structure for the annotations. If one developer decided to mark up movies one way, and another developer decided to do it a different way, none of their respective features would work properly because other applications wouldn’t be able to interpret them.

So Twitter’s actually giving suggested structures for annotations. They include annotations for reviews, songs, movies, books, products, stocks and more. The full list is here. The new guidelines should prevent a mess of different standards.

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