I'll have some Plone websites in my ZOPE server, something like twenty websites, and I need to make a search between all of the sites. It's for a university in Brazil. I tried using collective.solr but I couldn't find a way to make the multiple site search. Is there a way to do so? If yes, could someone help me with this? I'm using Plone 4.3.
Short answer: there is no solution for that.
Long answer: this is hard to achieve in particular due to different security contexts etc. The common approach for providing cross-site searches of arbitrary sites or heterogenous sites is to use a crawler or an external solution.
My slides
http://www.slideshare.net/ajung/gsa-goodbye
might be helpful (german only).
-aj
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Thank You, i'll try that 
There's a brazilian Django-based solution from Interlegis that lets you index any website that provides search results in RSS format. If it doesn't you can register a parser based on XPath/XSLT. Manuals and all in https://colab.interlegis.leg.br/wiki/MetaBus
Is there an add-on to use MetaBus directly from the Plone website? Or it will be something like a search portal website?
The typical requirement is that searches in a portal find only things in this portal. Therefore, "collective.solr" will likely try hard to ensure that only hits from the "current" portal are reported.
You could implement a kind of "meta search". Such a search interacts with the user to obtain the relevant query parameters, then dispatches corresponding searches to a set of search engines ("portal_catalog"s in the Plone case) and finally forms and presents the cumulated search response.
"portal_catalog" search results often depend on the authenticated user. This can complicate searches from outside the corresponding portal context. However, results visible for "anoynmous" or searching for "global" users (i.e. defined in a context above all portals) should work.
I'm not aware of an integration plugin for Metabus but you might want to display it in an iframe by using Products.windowZ or simply adding the tag in Document's rich text field (maybe you'll need to allow it in @@filter-controlpanel). Alternatively you could create a browser view to consume and display the results. You may contact the developers - since Interlegis is a heavy Plone user, they might have some recommendations: metabus@googlegroups.com.