Unless the original author(s) are sought out for permission or give it
themselves to release the game, then technically the items are still
under normal copyright.
For some actual legitimately free games, check here:
http://liberatedgames.com
Abandonware sites usually work on the principle of "It's ignored now by
the publishers, and no one cares" coupled with "We'll take it down if
someone asks".
There are also orphaned works - ones where it is hard to track down the
rights of anyway, which are always a pain.
Andrew