Do you use any of these technologies? Which ones are current and hence sensible to include in a site?
Documentation on any seems to be relatively sparse, and usage of any of them limited, as search engines get better, are they even relevant any more?
I use microformats whenever I can. Usually it just makes sense anyway, as frequently when I have an address block, I may want to style some elements differently then other elements, and that makes it super easy. It's not like microformats are that hard to figure out.
There has been a couple of instances where, because I was using a microformat, I was able to re-use the markup of a certain portion of a site somewhere else (as an include), and not have to change the markup, only the styling.
Finally, ever since I really stated exploring microformats, I got better at writing semantic markup and better at naming CSS classes.
I've used all three, but of the lot I'd have to say microformats have the most momentum these days. It has the advantage of being very easy to implement, even as an afterthought, on existing sites. And while there don't seem to be a lot of microformat consumers in the wild at this point, that situation is starting to change with the next generation of browsers.
As for the relevance of semantic markup in general, anything that makes it easier to automate the gathering of data is going to contribute to a richer ecosystem of applications that use that data. Relying on search engines for this kind of contextual processing does not address the needs of more focused or niche applications.
I use SIOC, FOAF, DOAP and some other lightweight RDF vocabularies. A popular trend these days is to embed RDF in web documents using RDFa. At that point the border b/w RDF vocabs and microformats starts to disapper.
With search engines getting better the need for semantic markup will rather rise. For an example, take a look at microformats and vocabs that Yahoo SearchMonkey apps can use: SearchMonkey vocabularies
Once search engines can make sense of richer data (even if at first it is just to display richer data about a match found) people will also get more motivation to use semantic markup.
What additional documentation do you think is missing and would be useful to have?
We baked microformats in the publisher tools we develop at Praized (mostly in the plugin tools we provide to bloggers)
Since the core Object in our system is "places", we thought it was sensible for us to output microformats.
I use RDFa as it has the important feature of being able to say anything, even really bizarre or obscure facts (such as the properties of archaeological finds, or the number of friends Paris Hilton has on MySpace), and doing so unambiguously.
I was recently working on a Search Monkey plugin to display VCal data embedded as RDFa, and stumbled upon a couple of cases where you just need that extra little bit of data to connect things. They were: connecting a presentation to the slides used during the presentation, and connecting a web page to its primary topic so you can tell exactly what the page is about.
Its difficult to see how you would answer those use cases with Microformats, there is neither context or precision in the markup. Over time I'll want to add more detail to my RDFa to help different groups of people find my pages and buy stuff.
Dublin Core is available in RDF and RDFa, but the old DC meta tags have similar issues to Microformats and even lower active use as far as I know.
I agree with CaptSolo that while DC and other meta data standards are old hat, RDFa is a growth area. RDFa.info chronicle each new user as it comes along. I'd go further and predict that microformats will quickly die off as more people 'get' RDF and more RDF-aware tools are produced.
At yahoo we support RDFa, eRDF, and microformats in page's markup.
To see what we have harvested, install my SearchMonkey plugin, and then do any search using Yahoo. You should see an infobar below showing the semantic data.
I can't post links since I'm a new user, but goto the SearchMonkey gallery, and look for "Structured Data Display". Its under technology.