TL;DR A website to show seeds, share them and find new ones with some biomes or structures you specify near spawn. All in your browser and even on mobile.
Seeder is a web app built upon a C library (cubiomes) that uses fancy new technology to look up seeds or find new ones based on some filter you provide. All whitout installing or configuring anything. It even works on mobile, pretty well I must say.
It's all still in beta, but the provided information should be 100% right.
Generates biomes for a given seed
Find the spawn point
Find a seed containing a list of biomes in a given range
Find a seed containing a strcuture in a given range
Find a seed containing a strcuture AND a list of biomes in a given range
Works with different MC versions
Finding rare seeds may slow down you PC while the search is running (seed finding is a very CPU intensive job, is this really a feature?)