You're free to use all the basic rules that have been made open source by WotC, which includes some prestige classes and additional rules from supplement books, and you can even change the ruleset with house-rules to your heart's content (like we're using hexes instead of squares, and we're thinking of adding some form of high ground bonus, etc).
This does not include any copyrighted creatures such as beholders and illithids, but it does include all generic monsters such as orcs, goblins, gnolls, dragons etc which you can freely copy from the old monster manuals.