DailyMotion is a great site to watch episodes of old wrestling. Managed to find old episodes of Raw and SmackDown from 1999-2000 and Nitro from 1998. Justin.tv has their moments of having episodes and documentaries.
there are a few streams on Justin.tv which show back to back episodes of WWE stuff, normally attitude era. There was one up which had every WWE and WCW raw/nitro and PPV in Chronological order from 1997 to 2001 (upto the conclusion of the WCW/ECW invasion). but unfortunatly that has long gone