Ask HN: How do you find interesting companies to apply to?

I monitor StackOverflow Jobs, HN Who's Hiring and a few language-specific sites already, but I'm aware that a lot of the most interesting jobs never go up on aggregators. I'm wondering if there's a good list of interesting small-to-mid-size tech companies that I could use to pick out some places I want to periodically check.

I'm primarily looking in London so ideally the site in question would have the ability to filter by location, but it might be worthwhile even without.