Have you got a partially assembled LEGO set that you're struggling to identify? You do? Then this web-based tool is for you!
As long as the few parts that you can identify originated from the same set, this tool has a pretty good chance of being able to guess what that set is. You may need to supply 3 or more different parts to whittle the possibilities down to a single set, depending how commonly used those parts are.
This is a beta service. I find it quite useful, and you might also find it useful if you use it the same way as I do, but don't expect it to give any helpful error messages if your inputs are silly.
Enter the Element ID of each part that you can identify (e.g. 6191995). An Element ID is a unique identifier for a part design in a particular colour - for example, the element number for a red 2x4 brick is 300121 (see https://brickset.com/parts/300121/brick-2x4). Do not use a part's design number, as this corresponds only to its shape and not its colour. Unrecognised elements will be ignored/removed.