Wikipedia
Access 
Based on a free, open and knowledge-sharing project, the entire content of Wikipedia sites can be freely read, copied and distributed (under the GNU GPL license). Cross references and links are available everywhere.
Fee Policy 
Faithful to its open and knowledge-sharing nature, any Wikipedia content is completely free of charge, and users are encouraged to post their entries
Free Retrieved Material
The whole material published in its different sites/languages can be retrieved without any restriction whatsoever
Linking Richness
Cross-reference and deep linking are among Wikipedia's main features. Hypertextual links are provided each time that a word, date or reference has been edited in the Wikipedia pages