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.
Simplest option
Use the Gargoyle app, which will launch most text games.
Gargoyle for Windows
Gargoyle for Mac OS
Gargoyle for Linux (?)
Other options
Other options depend on the game format, which you can usually tell from the file extension.
Z-code
(Extensions: .z3-.z8, .zblorb, .zlb, and sometimes .dat)
Windows: WinFrotz
Mac OS: (?)
Linux: (?)
iOS: (?)
Android: (?)
Glulx
(Extensions: .ulx, .blb, .gblorb, .glb)
Windows: Glulxe; Git
Mac OS: (?)
Linux: (?)
iOS: (?)
Android: (?)
TADS
(Extensions: .gam, .t3)
Windows: TADS Player Kit
Mac OS: QTads
Linux: (?)
iOS: (?)
Android: (?)