Listed hereunder are several files, created by gamers on the net, which work rather well with TSR's Advanced Dungeons & Dragons game (1st & 2nd ed.). Included are various resources, house rules, collections of spells/items/monsters, adventure modules, fiction, computer programs, humor, and a lot more. This isn't anywhere near an exhaustive list, but it is a pretty large sample, and should fulfill most people's needs in this area.
UPDATE (11/14/02): I no longer have time to track down all of the AD&D material out there, nor do I have time to regularly and frequently check the massive number of links in this and track down those that have moved, gone missing, etc. Also, with the explosion of d20 pages, fan pages, and so forth across the web, updating this to exhaustively include 3rd ed. material would be a nightmare (especially for one fan, working alone), therefore the likelihood that this will get updated any time soon is exceedingly small. Because people are still finding it, and apparently finding it useful, I'm keeping it available, but be warned that it's no longer exhaustive and there is an ever-increasing amount of link rot. If you can't find something here, or find that the link is dead, try Blue Troll's netbooks webpage for one possible--more up to date--alternative.
UPDATE (10/12/99): Once upon a time, MPGN would automatically uncompress files during transfer, if you requested the file without the ".Z" suffix. MPGN apparently no longer does this, but since I discovered that, I haven't had the opportunity to go back and add in all the Z's. If you come across an ftp link to MPGN that doesn't work, try adding a ".Z" on the end of the URL.
Please note that none of the listed files are located at this site, but rather are merely links to other sites. Also note that most of these files are for personal use only; they cannot be given, mailed, etc. to your friends (or enemies), or uploaded to web pages or FTP sites without the author's permission (often, but not always given in the file itself). Do you know of a net.book that's not listed here? Or a new version of a listed netbook? Please let me know.
MPGN, the "official" FTP site or
TSR's web page
The Great Net.Book Archive
Nostradamus' Giga Archive
ADnD.com's Net.Book Archive
Anime-Adventurers.com
All other web pages
A further note that not all of the e-mail addresses listed may be valid. Many of these are several years old, and garnered from the files themselves; it's very possible that an author/compiler has changed addresses several times since. For completeness, however, where I've found an address, I've included a mailto link; people are sometimes easier to track down if you have some starting point.
Net.Books
Spellbooks
Magic
Campaign Settings
Adventures
Monsters
Fiction
Programs
Humor
Miscellaneous