Searching for nodes from another site in a multisite installation I am trying to have the search function of a www.site.com return also nodes from site.com/somethingelse. Is there any way I can achieve this?

I prefer not to use Solr.
You could do that easy enough with Google Custom Search, e.g:


http://www.google.com/cse
http://drupal.org/project/google_cse


Basically can search a range of selected sites from one interface, using the power of Google search.