Online freely available cookbooks

This is an incredibly useful Hydrocolloid Recipe Collection if you are interested in modernist techniques (so called molecular gastronomy).

In my mind- the difference between a cookbook and a site that collects recipes is that I can typically trust the recipes in a book. I assume that they have been tested and are more reliable than some random recipe that I run across in someone's blog.

As such- I use allrecipes.com more than any other cookbook. (Though The Joy Of Cooking is a close second) The recipes are user rated and I have built confidence that highly rated recipes will be successful. In fact- I have more confidence in a recipe rated 5/5 stars by 1000 people than I do in the recipes from my better homes and gardens cookbook.

I also expect more from online resources than I do from a physical cookbook. If I'm collecting recipes online I fully expect the resources to allow me to search by desired (and undesired) ingredients, scale servings, allow me to tag and categorize, and even create shopping lists.

Only a small portion of their content is free, but many cooks I know find the price of a subscription to Cook's Illustrated/America's Test Kitchen/Cook's Country well worth it. These are online cooking magazines, but the parent publishes the same content slightly edited in book form, so in many ways they are an online book. You can find recipes, technique advice embedded in the recipes, and product reviews. Because of the style of Cook's Illustrated, the recipes have been honed over and over again until you can be quite confident in them. I own the America's Test Kitchen Family Cookbook, and find myself turning to the website at least as often as the book (and I don't personally pay for a subscription).

Also for online magazines you can find content from Bon Appetite and Gourmet (and others) at Epicurious. This is an entirely free resource. Cooks rate the recipes. I've had some successes here, but also some stunning failures.

I've found food.com (previously called recipezaar.com) the most reliable site so far. I use it constantly. There has only been recipe that wasn't my favorite so far and that was still decent. Like @Sobachatina mentions the actual cookbooks are tested so I thought they were more reliable but since food.com is free and open I can easily choose recipes by how many people have rated the recipe and what the recipe is rated as. So if I am looking for a recipe for chocolate cake I can easily sort the results to get the recipes with the highest ratings and the most ratings. Another benefit I've found is user comments. While looking over the recipe I can look at the comments that were submitted and benefit from their past experience so recipe turns out better. For example if several of the comments said that the dish was too garlicy and I don't like garlic then I know to decrease the amount of garlic.

Feeding America: The Historic American Cookbook Project is a lovely collection of online cookbooks.

About the project:

The Feeding America project has created an online collection of some of the most important and influential American cookbooks from the late 18th to early 20th century. The digital archive includes page images of 76 cookbooks from the MSU Library's collection as well as searchable full-text transcriptions. This site also features a glossary of cookery terms and multidimensional images of antique cooking implements from the collections of the MSU Museum.