Looks like Planet Planet added Atom support recently. Plagger also has a nice Planet publisher plugin Casey wrote for ETech 2006 and thanks to a nice design of Plagger plugin system, it's damn easy to add Atom/RSS/OPML output in combination with Planet output.
Today I added another fancy Subscription module called Subscription::XOXO. It tries to get XHTML and extract subcription list using XOXO microformats. This is pretty nice, because someone can update their source of "interesting links to Perl hackers" for example, and you can use that single XHTML entry as a source of your subscription.
Another very interesting plugin created these days is Search::Estraier (screenshot). It uses a pretty fast search engine Hyper Estraier and its P2P node API to index entries aggregated by Plagger. Nice example of usage of this module will be: 1)create your own Blog Search with Subscription::PingServer + Aggregator::Xango 2) aggregate all entries from the web containing certain keyword using Subscription::Planet, then index it for later search to be used as a Knowledge Base, like this Plagger KB.