I'm aware that in general, plugins are not available at wordpress.com. However, I'd like to suggest a plugin that I and others would find very valuable.
My blog posts are aggregated at FriendFeed (for example http://friendfeed.com/neilfws?service=blog). Most people prefer to comment there rather than at the original post and I get a lot of feedback. It would be nice to see that feedback at the original blog post.
An excellent plugin exists for this purpose:
I think many users of WP and FriendFeed would be very happy to see it made available.