OSRIC

 

The Old School Reference and Index Compilation is a set of fantasy role play game rules.  These rules recreate the rules and feel of RPGs from the 1970-80s.

These rules are free and great pains have been taken to remove any references to licensed intellectual property.  The link above will take you to the free download for the book.

If you are looking for the old school feel of Advanced Dungeons & Dragons, this may be the book for you.

In addition to the rules there are many on-line resources for adventure modules (both free and pay.)