An app that you don't have to buy, so its free. You buy video games on it, some are free and some aren't.
...steam is just a downloadable purchaser...
On steam
usually you just right-click the game then properties, then unistall
Lucasarts has released a downloadable version of RC on steam. http://store.steampowered.com/app/6000/
The downloadable Program and gaming network Steam is a good place to buy Terraria, although it is coming to console soon.
In order to play the DLC (downloadable content) "Zombie Island of Dr. Ned", you need the full main game Borderlands in your Steam games library.
The official promotional site for Civilization provides links to both the regular and the Game Of The Year Edition from Steam. By downloading the Game of The Year edition, you receive $40 of Downloadable content.
There is free downloadable content for both versions and PS3 has an exclusive free downloadable challenge.
Go and buy the game. If it's downloadable, one could purchase it over Steam or another service. If it's retail, head to a store and buy it there. Enjoy.
If one has access to a computer, downloadable games are readily available. Game House is a source of downloadable games. One could also get downloadable games from the Play Station Store.
Basically every website is non-downloadable.
I don't believe you can because the maps are only downloadable from the Steam Workshop, not the PS Store. I hope they have a way soon but for right now it is only for PC. If you bought Portal 2 new you should have a redeem code for a free copy of Portal 2 on Steam.