This article will help you find interpreters for MacOS X. After reading this guide you should be able to make a complete collection of interpreters for all of the major text adventure languages. Note that I won't be describing the different languages and virtual machines that are out there; if you don't know why you'd want a z-machine interpreter, you should read my article on the various interpreters first. Similarly, I won't tell you what the Interactive Fiction Archive is; if you don't know, read this guide to the IF Archive first.
As I've said before, writing instructions for downloading and installing interpreters is akin to trying to nail jelly to an air mattress. This guide will no doubt be out-of-date in short order. If you've found an error in this guide or if some of it is now out of date, drop me a line.
There are some OS X interpreters which come as disk images, recognizable by their extension .dmg. Occasionally they are gzipped as well, in which case the file ends in .dmg.gz. The nice thing about disk images is that you can mount them by double-clicking on them, making them behave just like a removable disk. Once you've done that, you can open the image and copy files wherever you want them.
Finally, if all you're looking for is links to the latest interpreters, you should skip ahead to the last page.