Concept
A website which lets you tell it what food you've got in your kitchen so that it can list recipes that use 'em. (Essentially, it would be an enhanced version of CookingByNumbers.com or The Reverse Cookbook).
Design
A brilliant aspect of CookingByNumbers.com is that it's amazingly simple to use; the list of food you pick from is quite short, and contains the most common foods you'd have in your fridge/pantry. It also groups herbs into a single category, which is a great idea.
This philosophy of simplicity should be adopted throughout the entire site -- it's best to trade-off powerful features and huge numbers of recipes in exchange for ease of use and a tractable recipe database (WITHOUT THOUSANDS OF REDUNDANT AND CRAPPY RECIPES).
Features
User accounts, for loggin'-in purposes.
Rate recipes, for better search results.
Bookmark your favorite recipes.
Cluster users by ratings and bookmarks to further enhance the quality of the ratings and search results.
Recipe calendar shuffler which randomly generates a week or month of your favorite meals (nutritionally balanced, of course). It can then automatically create a shopping list for the next x days. Also, you could have little controls to change how it picks meals -- for example, you could increase meal "variety" (ones you haven't had in a long time, or meals that have a variety of ingredients), or it could select based on the cost of the meals (for bums like me!).
More ingredient choices will be available, organized by categories. cookingbynumbers uses a minimal set of ingredients and groups them into "In your fridge" and "In your pantry". Maybe there should be more ingredients for more exotic recipes (asian cooking, vegan recipes, etc.). These would be in collapsed categories that pop open when you click 'em.
Hide ingredients which you won't ever buy or use. (An advanced feature for users with accounts.) This should be AJAX, and it should allow you to modify the list of ingredient choices directly, without a page-reload.
Printable recipes, just like allrecipes.com. It should let you scale the recipe by "number of servings".
Colour photos of each recipe would be amazing. Pictures of food are way easier to scan than names.
Ingredient icons in the search results would also make scanning the list of recipes quicker -- you'd see what was in each recipe without having to load another page. (eg. If you don't feel like anything with garlic tonight, you can easily ignore those recipes.)
New
Show what's popular in other people's fridges, in case you want some ideas on tasty ingredients that you weren't aware if (like tahini).
Show sub-components of recipes that can be made out of your food. If you don't have enough food for an entire recipe, or if you're an experimental kind of cooking person, you can see all the tasty combinations of things you've got so that you can formulate your own recipe! Also, this feature would show you recipes that contain that food sub-component (eg. you have the ingredients for a tasty cheese sauce, which these other recipes use). It would give you ideas for things you should buy to use up stuff in your fridge.
Partial recipes would show you recipes that you have most of the ingredients for.