The most popular and/or powerful, these are currently used by a large number of people; many posts to rec.arts.int-fiction concern these systems and their use; games produced with these systems are guaranteed a relatively large audience.
The parser can be supplied with a language definition file allowing Inform games to be played in non-English languages. Translations of Inform have been made into German, Spanish, Italian and Renaissance English, with several others in development. These translations are linked from Graham Nelson's website (see below).
A system called "Blorb", for convenient attachment of sound effects and modern-quality graphics, has now been fully implemented by Kevin Bracey's "Zip2000" interpreter.
An alternate version can compile to the "Glulx" format, which allows advanced I/O capalities and removes many of the ZMachine's arbirtrary size restrictions.
The latest (fourth) edition of the Designer's Manual in PDF can be downloaded from the IF Archive </if-archive/infocom/compilers/inform6/manuals/designers_manual_4.pdf>. An HTML version is also available (see next section 'Online Documentation'.)
(It may also be available as paperback book. Check <http://www.iflibrary.org/dm4> for the latest details.)
The Inform Beginners Guide is an excellent introduction to writing IF with Inform.<http://www.firthworks.com/roger/IBG.html>
As of Inform 6.21, you can compile your games with "Infix" debugging mode. This allows the author to use a variety of debugging verbs to examine and change the game's state in a way similar to the programming of Inform itself. You can trace references to a routine or object.
Inform can also produce a file of information useful to any debugging tool, with, for example, Z-machine PC positions assigned to every statement of source code.
There are some help-tools to configure various text editors to Inform, too; the Technical Manual provides an algorithm for syntax-colouring Inform code which is used in several of these.
TADS allows you to write your game using HTML TADS, which allows you to add styled text, still graphics, sound, and MIDI music to your TADS games. Rather than relying on some proprietary markup language, HTML TADS uses standard HTML, the language used to mark up Web pages, for which documentation is readily available. (However, HTML TADS doesn't need a web browser or the Internet: it uses HTML but isn't a Web-based system.) HTML TADS also supports ISO Latin-1 character sets, so accented letters are easily added.
At the time of writing, HTML TADS interpreters are only available for Windows 95/98/NT and Macintosh. However, a game written in HTML TADS is still fully playable (minus graphics and sound, of course) with character-based TADS runtimes that have been updated to at least version 2.2.6.
The Windows port of TADS 2.5 and up comes with "Visual Workbench", an integrated development environment including the compiler, debugger, runtime, and an editor. It can even create self-enclosed executable installater programs for Windows for your TADS games.
In addition to the manual the full source for a medium-sized game, Ditch Day Drifter, is available from the usual sources. The source code for many other games, from small and simple to huge and complex, is also readily available.
Mark Engelberg has written a tutorial for TADS. It can be found on the IF-Archive as /if-archive/programming/tads2/manuals/TADSTutorial.zip .