Getting started playing IF, it’s a given you’re going to need an interpreter (at least until there’s a way to make executable files for every platform in your authoring system of choice!)
There are several options here, but the ones I use are as follows:
Gargoyle, for Windows. Found at ccxvii.net/gargoyle/
Gargoyle is an interpreter that has included the following interpreters in it’s package: Agility 1.1.1, Alan 2.8.6 and 3.0a1, Frotz 2.43 (glk port), Glulxe 0.3.5, Hugo 3.1, Level 9 4.0, Magnetic 2.2, Scare 1.3.3, Tads 2.5.9 and 3.0.8. That means it will play any of the games those interpreters run.
Spatterlight, for Macintosh. Found at ccxvii.net/spatterlight/
Spatterlight is an interpreter, similar to Gargoyle, that can run AGT, Adrift, AdvSys, Alan, Glulx, Hugo, Level 9, Magnetic Scrolls, TADS (text-only), Quill, and Z-code (except v6) games.
Those two interpreters will play most of the games you’ll encounter (although sometimes without the included graphics, if they have any.) Usually around competition time, I’ll use more full-featured interpreters so I can see and hear everything the authors included in their games.
There are plenty more out there, so let’s hear what you use.