Basically I'm looking for related posts between websites without using any API or direct access to the databases (direct parsing of the content perhaps?)
I'm guessing an open source search engine (preferably coded in PHP) would do the job. Did anyone work with anything similar or has any recommendations?
So after a bit of research I ended up with two possible solutions:
http://www.sphider.eu/ - php search engine and crawler. I guess it can be modified to fit my needs.
http://www.bing.com/developers - Using the BING api to get related posts. Basically I'll have to extract a search query from the current post, pass it through BING and get my related posts.
I couldn't find an open-source alternative that dose exactly this so maybe someone that has more time on their hands are up for a challenge :)