There are a few websites that can give you a list of recipes if you tell them what ingredients you have in stock. These are okay, but it's inconvenient to have to a) look in the fridge and pantry, remember or write down what's there, and then match each item to the checklist on the website, with most of what you have not being on the website's checklist and therefore a waste of writing; b) write down the website's list, go check off what's in the fridge and pantry, and then go back to the computer to transfer that info to the website, c) take the computer with you to the kitchen and check things off on the website as you look in the fridge and pantry, or d) make multiple trips between the kitchen and the computer to check for a few things at at time, as many as you can remember. You could of course have a phone/tablet app, or a phone-optimized website, instead of a desktop-optimized website, and do option c more conveniently using that, but I haven't seen that available (not that I've looked) and it would still be a significant manual task, especially if the checklist is extensive.
A more convenient way would be to just point your phone or tablet's camera at your fridge and pantry, wave it around and move/turn things with your other hand so it gets a good view of everything, and let it figure out what's there on its own. That should be approximately feasible nowadays—it seems like technology to do similar things is well available, e.g. Apple's ARKit and Google's Project Tango and Google Goggles/Lens.
Once the app knows what ingredients you have, and you've told it what kind of food you're in the mood for, (and knowing the answers you gave to its new-user questionnaire about your preferences and dietary restrictions, etc.,) it can then suggest recipes. But this app is not just a traditional recipe-from-ingredients search engine plus computer vision. It also has a recipe generation system built in, which uses machine learning from human-written recipes and a database of what ingredients are known to taste good/bad together (which already exists in some form—see The Flavor Matrix, which, incidentally, was created with the help of IBM's Watson) to come up with novel recipes that are predicted to taste good and can be made with the ingredients you have. It can also suggest recipes with a few additional ingredients, in case you're willing or planning to go grocery shopping.
After you make a recipe it suggested, you can tell it about any deviations from the recipe you made and what you and the other eaters thought of the result. It will use this to tailor its recipe suggestions for you in the future. If you tell it who's eating, it will tailor its suggestions based on what they've thought about previous dishes.
It can also take allergies and other dietary restrictions into account, and suggest alternative recipes or automatically substitute ingredients in its recipes. As well, you can show the app what kitchen appliances and tools you have, and you can tell it what cooking techniques you know, and it will take these factors into account as well.
N/A [2019-10-08]-- notexactly, Oct 10 2019 Same old database, good data, problem via camera magic. Got to use the stuff at the back of the cupboard first.-- wjt, Oct 12 2019 Scan every ingredient when you buy it/put it away in the cupboard.Then your database knows what you’ve got, what needs to be used up now, and what you’re running low on.-- Frankx, Oct 12 2019 random, halfbakery