There are thousands of web pages with "Pyrotechnics Information" available. All of them are much better than anything I have here. Unfortunately most of the web pages you'll stumble across on the web are full of dangerous misinformation that will kill you if you're lucky and simply remove parts of your body if you're not.
I have to believe that many of the idiots that post garbage from The Jolly Roger Cookbook, The Terrorist's Handbook, etc have removed their genes from the pool but the crap they spread lives on. The Internet has been an information treasure to pyrotechnic hobbyists but unfortunately the garbage put out by these fools spreads just as fast. I strongly advise double or triple checking any information you find.
If you're new to the hobby, you might find the following links helpful. They are in no particular order except for the first one.
SAFETY
Whether you're new to this or an "old-timer", please click these links. Read the pages, print them out and read again.
Skylighter's article on safety
Fireworking.com A fantastic, subscription based, ($40) online resource hosted by Ned Gorski. Ned has written many tutorials for Skylighter and brings that expertise on a full time basis to Fireworking.com. Along with the articles on different aspects of making pyrotechnics there's also fireworks project plans, discussion groups, formula database, & market. It's a friendly atmosphere and almost like sitting around a campfire. Pull up a log. crack a cold one, and absorb the years of knowledge that's freely given. Ned's motto is "Come for the information, stay for the conversation".
Passfire.com Another online resource for the amateur fireworks builder with high quality articles, illustrations, formula database, and forum. The subscription rate for Passfire.com is $40 for first time subscribers and $25 for renewing subscribers. A guest account is available for a limited account that remains active for one week. I cannot stress enough how valuable this site was. Unfortunately it is no longer maintained and there are very few new postings.
Wichita Buggy Whip This site contains information you need for Black Powder based devices and compositions. One of the best BP rocket sites. Before making any composition using charcoal, you really need to check out the tests these folks have done on different types of wood.
Dan Williams Page Chock full of tutorials and projects. Also allows user comments and feedback. Excellent site for the beginner as well as the "established" hobbyist. This site has been referenced often over the years but difficult to find and hosted on different servers. Dan has resurrected it and seems to have found a permanent home. It will be fun watching this site grow.Update Once again Dan's page had left the building. If anyone finds it, let me know and I'll link to it again.
Skylighter.com Online supplier of virtually any pyrotechnic chemical you'll ever need. In many cases, Skylighter is the only source for the casual hobbyist. They have rather high prices but are very dependable and consistent in quality. The best feature they have is their online newsletter archive. Dating back to 1999, their newsletter contains hundreds of useful tips, tricks, and tutorials. Well written, informative, often humorous, and FREE.
CannonFuse.com Another online supplier with pyrotechnic formulas, tutorials and reference. The chemicals available are no where near as extensive as those available on Skylighter but are often much cheaper.
Firefox Enterprises Online supplier of chemicals and supplies. Their prices are lower than Skylighter's but they add a mandatory 5% contribution to the CSPC defense fund and can be expensive when it comes to shipping. In the past, they've had a less than stellar reputation when it comes to returning phone calls. Calling seems to be the best option. UPDATE: The CPSC has severely restricted the amounts of supplies this company can sell.
APC Forum Amateur Pyrotechnics and Chemistry forum is a general information-sharing space for people who are interested in pyrotechnics. Contains member contributions covering compositions and pyro techniques. Warning! Very tightly moderated and I strongly suggest you read the rules before trying to post here. Scroll down and read the very informative supplier reviews.
UK Pyrotechnics Society Another well run online forum with very good information. Since they're geared towards UK users, you may have to do some conversions. ( Hint; 2.54 cm = 1 inch )