If you don't know it yet Portal : Prelude is a (very) tough mod for Portal.
And if you don't know Portal, it is an amazing puzzle fps.

I found a way to play it without having full Portal game. Nothing revoltionnary but nice to know

Here the step you need to be able too :

  • Download the mod at official website : Portal: Prelude
  • Install it.
  • Edit the file : C:\Program Files\Steam\steamapps\SourceMods\portal prelude\gameinfo.txt
  • Change the line :

        SteamAppId                400
        SteamAppId                410
  • Launch the Portal Prelude and let steam install Portal Demo or do it yourself at Portal: First Slice on Steam while downloading the mod at first step.
  • Play the mod ! (though I suggest you play demo first)

You are now playing a mod of the demo
The demo is actually the 11 first levels of full game.

Not sure there are other worthy mods for other games, but you might apply similar tricks to cheat steam again in such cases.