The best site for this stuff is
www.the-underdogs.org . Its recently had to change URL (cybersquatters, gotta hate em) so search engines dont show it up any more.
If the game you want is not there then it might be on one of the sites they link to, or it might be a game the original authors has specificaly announced is not for free download. In that case its *definatley* illegal to download and you should stop looking for it *now*.
On that note, anything by lucasarts (ever) is illegal to download. To my lasting regret. Some companies are more friendly though.