How to use urnc.yi.org:

The simplest way is 
http://urnc.yi.org/<URN>

Example: 
http://urnc.yi.org/urn:sha1:IMCWGRYKZDCGNMC3KUT3MCFMNDM3VPXR

You can also use the boring RFC-compliant way: http://urnc.yi.org/uri-res/N2R?<URN>
Example: 
http://urnc.yi.org/uri-res/N2R?urn:sha1:IMCWGRYKZDCGNMC3KUT3MCFMNDM3VPXR

You can specify the order in which the various URN resolvers are checked: http://urnc.yi.org/uri-res/N2R?<URN>&order=<ORDER>
Example: 

http://urnc.yi.org/urn:sha1:IMCWGRYKZDCGNMC3KUT3MCFMNDM3VPXR&order=macro,tma

The default order in which the resolvers are checked is: cache, meidokon, tma, a4o
urnc.yi.org sends a X-Resolver header along with the requested resource, so you can figure out where it came from.

If you are using Firefox 3.0, you can set up your browser to handle urn:sha1: links directly: Install URN handler. You'll get a bar asking if you'd like to install it. Once you've accepted, you can use bare urn:sha1: links like so: urn:sha1:IMCWGRYKZDCGNMC3KUT3MCFMNDM3VPXR