Free LSI services or API to get related keywords

Question

I've been told that Yahoo used to had a free LSI service known as Yahoo Boss API that begans to being payed since July 20th and that Microsoft Bing Search Engine have a free service that offers similar but not as good functionalities as Yahoo Boss API.

Does there exist other free APIs than the above that provides such service to developers?

I've been trying to find something similar from Google without any results.

Answer 1

Unfortunatelly, as of Aug 2013 there are no such trustworthy services :( or at least I wasn't able to find anyone.