Killer app? Hard to say. There are many fun things to do here. In my limited spare time I write software that tests eepsites to check what can be found. I wrote it a while back as it was difficult to find working sites. Frustrated with this, a solution! http://notbob.i2p

It's not a killer site by any means. But, it lets you find content on the i2p network with ease. It only lists sites that work.