Robin Good: If you are curating a special collection on services and tools in a specific industry, you may appreciate being able to find out easily other companies and services that operate in the same niche.
While I would tipically search for popular keywords relevant to that niche to find all the relevant companies in that space, a much faster and effective approach is the one of using one of the several search engines designed just for this very task.
Yes, you have read me right. There are now more than a dozen of search engines dedicated to distill for you sites, services and companies that are most "similar" to the one you suggest.
For this reason I have put together a small reference map highlighting these specific search engines and linking to each one of them.
Full map: http://bit.ly/similarities-searchengines
(If you know of other ones please do not hesitate to let me know in the comments)