This thing about pills having coke or heroin mixed in is a bit of a myth - a dealer would have to charge 3x the normal price for pills like that. They'd be far too dangerous for a drug naive person to ingest - and drug dealers have no vested interest in killing their clientele. The danger of "speedballs" (coke or amphetamine + opiate) is well known and has killed many people, well publicized cases being Layne Staley and Chris Farley, if I'm not mistaken; and the danger of mixing coke and amphetamine should be fairly obvious.
Ecstasy (MDMA) is Methylenedioxymethamphetamine and the pills we've come across usually only contain this and binders, with different levels depending on the source, ranging from the very low, to the very high - which is why we always advise EVERYONE to only try half a pill when obtaining a new "brand" or buying from a new source.
Sometimes (though rarely) amphetamine or methamphetamine is sold as ecstasy, whether on purpose or by mistake.