Search for words that are closer to X or Y quickly
There is a popular site that asks you yes/no questions to guess a person you were thinking of in their large database of people ranging from actors to animation cartoon characters to famous scientists.
The way their database works is through a specialized binary search algorithm which takes your yes/no/not sure answers and narrows down the person you were thinking of. (Every time you answer yes/no ~50% of the characters are eliminated making the search very quick).
The idea is this: Be able to search for words that are at the tip of your tongue by answering whether it is closer to X or Y word (or not sure).
An example of this in action would be trying to find the word "apathetic" because I forgot it.
I would start with searching "bored" and the engine will ask me a question such as "Is your word similar to 'disinterested' or 'tired'?" and by answering "disinterested" it can then ask something like "Is your word similar to 'apathetic' or 'neutral'"? and from there I could click apathetic and mark it as "That's my word!".
The prompts that show the X or Y questions could have suggestions below the two choice that could be other options as well as the not sure button to find other words to search through.
This feature will be useful for those searching for words that they know they know but simply can't find. It is also easy to implement since your site already finds synonyms so it can easily map the relation between words.