I bought Full Throttle not too long ago... I got it in a multipack of various LucasArts games (that is still in stores at least in the UK) including Sam & Max, The Dig and Grim Fandango for about £20... pretty amazing value, even though I already had GF 
Still, abandonware is definitely illegal... There's an intellectual property issue that basically means that if companies don't 'try' to prevent abandonware on their products then they can be exploited by anyone... This means someone could sell a product based on their original work. This is why they all cooperate with some IP company, I can't remember the name...
Actually I believe it's the licence to play the game thats the issue... as far as I'm aware, downloading the game isn't illegal, but playing the game without a licence from the developer (either by buying the game in the shop or from a direct licencing agreement) IS illegal. It's not that developers don't want people playing their old games, and more recently it's become viable for devs to distribute their old games themselves and grant licences this way over the internet, such as Codemasters who I believe are doing this kind of thing, and closer to home (i.e. adventure games) companies such as Revolution Software (with Lure of the Temptress and Beneath a Steel Sky - both available from their website).
Hopefully more companies will follow suit, so they won't have to worry so much about appearing to be overprotective over their works, and still get it out to the people who they want to have it... the gamers.

Still, abandonware is definitely illegal... There's an intellectual property issue that basically means that if companies don't 'try' to prevent abandonware on their products then they can be exploited by anyone... This means someone could sell a product based on their original work. This is why they all cooperate with some IP company, I can't remember the name...
Actually I believe it's the licence to play the game thats the issue... as far as I'm aware, downloading the game isn't illegal, but playing the game without a licence from the developer (either by buying the game in the shop or from a direct licencing agreement) IS illegal. It's not that developers don't want people playing their old games, and more recently it's become viable for devs to distribute their old games themselves and grant licences this way over the internet, such as Codemasters who I believe are doing this kind of thing, and closer to home (i.e. adventure games) companies such as Revolution Software (with Lure of the Temptress and Beneath a Steel Sky - both available from their website).
Hopefully more companies will follow suit, so they won't have to worry so much about appearing to be overprotective over their works, and still get it out to the people who they want to have it... the gamers.