How to launch an IF game file
Many IF games can be played in a web browser, but if you download the game file, you'll need a launcher app (also called an "interpreter").
Below are download links for the most popular apps.
Use the Gargoyle app, which will launch most text games.
Gargoyle for Windows
Gargoyle for Mac OS
For Linux users, the first place to look is the main repository for your distribution. If you're using Linux Mint, open the Synaptic package manager and search for Gargoyle.
Other options depend on the game format, which you can usually tell from the file extension.
(Extensions: .z3-.z8, .zblorb, .zlb, and sometimes .dat)
Mac OS: (?)
(Extensions: .ulx, .blb, .gblorb, .glb)
(Extensions: .gam, .t3)