Can I play Steam games without running Steam in the background?

Is there a way I can play without having Steam running in the background?

There is no way to do so. Even if you navigate to the folder where the game is installed and run it directly, it will start the Steam client.

The closest you can get is to use the offline mode:

Offline Mode allows you to play games through Steam without reconnecting to the Steam Network every time you wish to play - this is particularly useful if you do not plan on playing over the internet and would prefer not to download new updates for your single-player games.

Please note that you must connect to the Steam Network and test each of the games you would like to use in Offline Mode at least once to set up your account and configure Offline Mode on your machine.

If you go to the Steam folder and try to launch the game from there, it usually also launches Steam together with the game. There are few exceptions, though:

• DOSBox and ScummVM games can be launched from outside Steam.
  • In case it doesn't work, at least you should be able to download and run DOSBox or ScummVM by yourself, and then load the game data from the Steam folder.
• Some Flash games might also be launched without Steam, but that's not guaranteed.
• Bora user mentions that you can run all Quake games without Steam.
  • Quake I uses DOSBox.
  • All Quake games have had their source-code released. So, in case they start requiring Steam in order to run, you can get a Quake port or compile your own Quake and run it, while grabbing the data files from the Steam folder.
• Maybe a few other games...

Update: I tried grabbing the data files for a few point-and-click games in order to run them inside ScummVM, but is seems the format is slightly different and they do not work outside Steam.