Ask HN: Best way to research companies?

What are good, hopefully free / low cost ways of getting basic and detailed information about companies?

For example, I'd like to be able to quickly and easily lookup the size (revenue, number of employees, etc) and location of companies that we're talking to or exchanging emails with.

I'm also looking for ways to research companies based on location / profile. For example, show me all companies with greater than 50 employees in the manufacturing business in region x.

Mostly for US and Canadian companies, but international would be great as well.