Cheers
The projects there have been described in my corresponding posts about them: http://arclanguage.org/submitted?id=fallintothis.
There's also the community-wide repo dubbed "Anarki" for alterations to Arc: http://github.com/nex3/arc/.
A lot of posts here are about people's projects, so have a poke around (e.g., aw's stuff at http://awwx.ws/ or evanrmurphy's http://tryarc.org/).
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http://tryarc.org/ is what I've been working on lately.
Also, you may already know this, but both this forum and http://news.ycombinator.com/ are Arc webapps.