Below you will find web sites and other resources that I find useful. If there is one that you think I should add, please feel free to drop me a line and I'll check it out!
Aquatic Gardeners Association: This is an international group for people who enjoy working with planted tanks and aquatic plants. Please consider joining our group. It is not expensive, and member benefits include a beautiful, quarterly, full color magazine full of articles that you will not find elsewhere. There is also a member's only section of the website, where you can access videos of many speakers from past conventions and other special content.
Aquatic Gardeners Association Face Book Page: We have over 9,000 members on the AGA Face Book page, and whether you are a paid member or not, you are welcome to join us. Ask questions, share your knowledge or share photos of your tanks and plants!
Flowgrow: This is a German website, but can be accessed in English as well. It has the most complete and most accurate on-line information on aquarium plant species that I know of.
Aquatic Plant Central: For those who like forums rather than Face Book, this is a nice, smallish forum with knowledgeable people.
The Barr Report: There is a tremendous amount of useful information on this site, and it has always been carefully moderated. There are some areas of the forum that are subscription/fee based, but other parts are free and open to the public.
UK Aquatic Plants Society: Although this is a UK based club, the website is full of useful information as well as a very nice forum.
Green Leaf Aquarium: There are other on line sources of equipment for planted aquariums, but Green Leaf is great for one-stop-shopping for CO2 equipment, tools and fertilizers. Lots of good information and greta customer service too.
NilocG: This is the home of "Thrive", arguably the best all-in-one fertilizer available. Colin also sells a wide range of fertilizers, CO2 equipment etc.
Greater Washington Aquatic Plants Association: GWAPA is a local group, but they have a tremendous knowledge base. They hold monthly meetings, but even if you aren't local, their web site has some very useful articles, and a well managed forum.
Chicago Aquatic Plants Society: CAPS is a relatively new organization, but this group is HOPPING! If you are in the Chicago area, you will find an enthusiastic, knowledgeable group to help you get started right! Besides their (linked) website, they have an active FaceBook group.