This is nothing new, which is why it's such an appropriate subject for a cutting edge blog such as this to write about after over 4 months of total silence.
An example would be review and price range information for a restaurant. Something Google currently provides on Yelp search results - one of their partners when they initially embarked on rich snippets.
How this works is Yelp.co.uk presents the review and price information on each restaurant page in a format that Google can understand using 'microformats' or RDFa markup.
In English that last sentence translates as; Google knows that the two dollar signs refer to the 'pricerange' and the 15 reviews refers to the amount of reviews (or 'count').
By defining the content of a website into principle parts (such as 'pricerange' and 'count') it allows for easy comparison across websites.
Instead of having to go into a website to see that Yelp has 15 reviews and Toptable has 20 reviews you can see this in the search results, such as in the screenshot above.
This could potentially work for any data. Imagine searching on Google and instead of just titles and descriptions you get a summary of a site page - so if it's a product, you see the price, if it's a restaurant it gives a rating, if it's an event, it gives a date.
This would make it easier for you to know the content of a website and quicker for you to search and surf.
Personally i think this is a direction the web could take IF websites catch on to this method of coding and agree specific microformats - for example everyone uses 'price' for prices as opposed to 'cost', 'amount', 'payment' etc.
The well established Google product search (previously Froogle) is making good use of microformats and Wolfram Alpha had a good bash at it last year (what happened to them?!*) - i definitely think it's an area to keep an eye on.
*Update: They are providing factual answers to Apple's Siri software amongst other things...
The only thing standing in the way would be cleverer search algorithms that can figure out which part of web page is the price, the review, the date without the need for it to be hard coded as such. I'm sure the magical web boffins are thinking about this as we speak. Shh, they can't be disturbed...
Search images by the colours within them (Feb 09)
What content types can Google Search? (Dec 08)