How does one find recipes given an ingredient rather than the recipe name?
This site has tons of good recipes. You can include up to 4 ingredients, pick a category, and enter additional keywords.
Supercook's core feature is a "pantry inventory" system.
You basically fill in all the ingredients you have in your kitchen. You are presented with recipe choices based on a subset of your ingredients, as you enter them. You can then "emphasize" certain ingredients which makes them a required ingredient in the recipes you are shown. It will also show recipes where you are only missing 1-3 ingredients; useful if you don't mind running to the grocery store. You can also exclude specific ingredients from recipe results.
Creating an account permits you to save your ingredients as well as your favorite recipes. You can also generate shopping lists based on the missing ingredients from your favorite recipes that you don't currently have in stock.
The quality of recipes varies a lot. Supercook sources its recipes from many other sites, including: Allrecipes, Recipezaar, Epicurious, FoodGeeks and others. Some sites (like Allrecipes) I feel have a lot of "filler" recipes, which are simply present for search results.
Epicurious doesn't provide the "pantry inventory" aspect that Supercook does, but it is very useful in its own way.
With Epicurious you search for your main ingredient, e.g. chicken, and then refine your search using ingredients and other parameters such as: meal, dietary requirements, holiday, cuisine, and preparation method (among others). I find their interface very intuitive.
One of Epicurious' greatest strengths is the quality of its recipes. Many of the recipes are originally from the reputable food magazines Gourmet and Bon Appetit. I find the recipe ratings to be very accurate, and the community feedback on the recipes is also of a reasonably high caliber. I also find their original content recipes to be very high quality. They often come up with quality themed recipes for major holidays, including full menus.
They also have a useful iPhone/iPad app that provides recipe syncing. I often use the iPhone app when grocery shopping to check off ingredients that I need for whatever dish I'm planning to make.
Allrecipes is another site I have used.
Allrecipes has a general purpose recipe search, similar to Epicurious. It also has an ingredient search that is similar to Supercook. The ingredient search doesn't provide the ability to save your pantry inventory like Supercook does, but is more of an ad hoc way to find recipes based on a list of ingredients and exclusions.
Unfortunately, I find the recipe quality to be rather low here. There are a lot of user submitted recipes which tend to range in quality from mediocre to atrocious. The feedback left by the community is also subpar. I tend to use it when I want a really basic recipe to use as a base for something more. They do have a lot of these "filler recipes", as I call them, which are very simple and often missing a thing or three that make them memorable. This is useful for experimenting or giving yourself ideas.
Food Network is another site I use that functions very similarly to Epicurious.
Food Network has a general purpose recipe search that permits filtering of results by cuisine, ingredients, technique, show, tv personality, and others.
I use Food Network primarily to look up recipes that I've seen on a Food Network show before, but didn't record at the time. I also find it very useful for finding new recipes by a chef I like. It's pretty much the canonical source for all Alton Brown recipes for example.
The community feedback and recipe ratings are middle of the road. I find they are less reliable than Epicurious, but far more reliable than Allrecipes.
http://www.recipepuppy.com/ seems to generally be quite good.
google recipe search! http://www.google.com/landing/recipes/
The main suggestions are provided above. But I'll add that you can go to just about any of your favorite cooking blogs. Many have a search feature and you can type in an ingredient name and a list of entries including that ingredient will pop up. I've tried this before.
But I do love allrecipes.com because you can add a list of ingredients.
What Could I Cook? is a great and growing resource of member-submitted recipes. One of its big supporters and submitters is the UK Guardian website, which attracts high-profile chefs.
Quite a few dead tree cookbooks have indexes ordered by main ingredient.