When I wrote the first edition of the Guitar Effects FAQ, there were no effects schematics sites on the world wide web. In fact, there was no world wide web, only the usenet news groups. It was only after the introduction of the graphics oriented net that there were any schematics available on the net at all. 

The first schematics web site was set up by Jamie Heilman (aka "Leper") as "Leper's Musical Microarchive". It's still there, but has been in hibernation for about years now. That site was the outgrowth of a collective of effects nuts that communicated through email and swapped schematics by snail mail. As the site grew, it ate up more web space at the college server that housed it, and chafed under the size restrictions. Jack Orman requested, and after some consultation between Jamie and me, was granted permission to post the whole mess at his web site, and later all the schematics were put on the schematics repository at ualberta. As the web grew, so did the number of places that posted schematics, and especially the places that simply copied all the files and re-published them. Today (late 1999) about half of all the schematics available on the web are still the same ones from Jamie's archive. There are some new ones, as well.

However the number of places to look have grown hugely. In addition to GEO, you can find schematics at:

Leper's Musical Circuits Archive

This is the first online collection of effects schematics, and probably the largest. Put together by Jamie Heilman

Guitar Related Circuits page - 

This site archived most of the schematics from Jamie's site. There is a mirror site as well.

Aron Nelson's DIY Effects site

One of the more complete sites available

Robin Tomtlund's Site

Swedish schematics site - the schemo's are in English

Stellan's Schematics

Another Swedish site, good schematic collection

JD's DIY Projects for Guitar Players

Schematics and beginner information. JD has proven himself a good writer of this kind of stuff - AND he uses initials, too! 8-)

Plate to Plate

Tube Screamer and Fuzz Face mods, other schematics.

GM Arts

Articles on distortion, amps, a few schematics

Justin Philpott's page

Schematics and beginner info

Frank Clarke's Zone of Distortion

Mods for the Tube Screamers and and some others.

CJLectronics

A smallish collection, but schematics you won't find other places

Swap Site for Guitar PCBs

Some layouts for effects PCBs

Electronics for Music II and Tom's Synthesizer Cookbook  

Primarily oriented to analog synthesizers, but with some guitar effects

Analog Modular Synthesizers

 Primarily synthesizer circuits, some guitar effects as well.  Most are in .pdf format and need Acrobat Reader to view them.

Paia Electronics

An old line effects/musical electronics supplier.

Books

1. Electronic Projects for Musicians by Craig Anderton
2. Electronic Projects for Guitar by Robert Penfold
   ...which outline not only the schematics, but how to read them and how to find parts, how to make boards and packages and boxes, etc. If you have no experience with effects, get one of these books first. In fact, get them anyway. They are very good references for people interested in effects.

Magazines
1. Guitar Player
   Other schematics are available commercially in back issues of this magazine, where Anderton wrote an irregular series of articles starting in the seventies in which he would outline an effect and how to build it. Some of these are significantly different from or more flexible than commercial effects.
2. Others ...
   Other electronics-related buildit magazines have published articles on the odd effect or two over the years. These include Popular electronics, Radio-Electronics, Electronics Technology International, Electronics Today, a little-circulated rag called Poly phony, and another called Electronotes (although the Electronotes articles are primarily oriented towards analog synthesizers).

Kits
1. PAiA
   Effects kits are available from PAIA Electronics, 3200 Teakwood Lane, Edmond OK, 73031, 405-340-6300. It is useful to write these folks and request a catalog if you're interested in effects. John Simonton is the president at PAIA, and is on line at PAIA.aol.com, or was for a while.