The Z-machine preservation project, a new Z-code interpreter (Z-machine) written in Java 5. Notable for its goals of clean implementation and reference-quality compliance to the standards, and for the ability to use it as a browser applet (for hosting or demonstrating interactive fiction on a web page). It's not the only Java applet Z-machine implementation, but it's new, it's maintained, and it's making good progress.

On the subject of Z-machines in browser windows, Muttonate also deserves a mention. Written in JavaScript, it's more an impressive feat than a usable implementation, as it is too slow to play some games in some browsers. The differences in speed with the different browsers are interesting; the author mentions some stats. (Says ChicagoDave in the announcement thread, "This is about the cleanest code I have ever seen for a z-terp." So perhaps this is also of interest for study.)

The Z-machine inspires many pet projects, despite the widespread availability of quality, free interpreters. Writing your own virtual machine that conforms to a retro standard and can play hundreds of games new and old is a seductive geek project idea, one I've considered for myself on multiple occasions. It seems the IF development community is half composed of people who have never written a game, but hope to find education and fun in developing support tools for authors and players.