How do I find the ID for a game on Steam?

Go to the game's store page and check the URL. The last number in the URL is the application ID. All the store URLs are in format store.steampowered.com/app/APPID, so for Wasteland 2, the URL is http://store.steampowered.com/app/240760/, and appID 240760.

Another reliable way is to search for the program on steamdb, which will show you the info for games removed from the store as well. The appid is clearly labelled in the search results.

Many of the methods above work but I am going to show you an offline version. I am using Windows and uncertain if it works with other systems. If you head into your Steam directory and open SteamApps, and then open common. There you will find a list of games. Open your chosen folder and find "steam_appid". Open it, and it will show the ID in a text document.

A list of all your installed steam apps and their appID's can be obtained easily with one line of BASH offline using grep, sed and awk to look at the appmanifest files in Steam/steamapps/(on Linux/Unix)

your path to /steamapps/ may vary if you install your games to a non default place.

grep -n "name" ~/Steam/steamapps/*.acf | sed -e 's/^.*_//;s/\.acf:.:/ /;s/name//;s/"//g;s/\t//g;s/ /-/' | awk -F"-" '{printf "%-40s %s\n", $2, $1}' | sort

Breakdown of what that one line of bash does:

1. grep for lines containing "name" in all the .acf files in steamapps/ the lines it returns are the filepath/name of the .acf with the appID in them followed by our mathed grep pattern "name" and the name of the application.
2. sed to strip the line up to and including the "_" (leaving the appid), replace the tabs with a single space, remove the double quotes, remove the word "name", replace the first space after the id number with a "-" (a character that is not found in any game names)
3. awk with "-" set as the delimiter to print the columns formatted with the name of the game followed by the ID number
4. sort to alphabetize