The Wordtracker Inspect tool is the only SEO tool that I know of to use actual Natural Language Processing technology to give you keywords. It works fantastically well at uncovering closely semantically linked keywords that other tools just could not see.
However for a while we had to take it down. To understand why, first I'll quickly walk you through how we develop tools. We pull in data from as many sources as possible, and the way we do this is through APIs. This is a way for a third party to share data with us. We will pull together and mash up the data from multiple API's and sources for a tool like Inspect. It's when you start mashing together data from multiple sources that you start to get really interesting useful tools.
We built Inspect using 'API up' principles, so we find the different data sets we want to use, and the really interesting combinations, and then build the tool around these. Where there isn't an API to do what we need, such as the complex page analysis that happens in the back end of Inspect, we build the service ourself and create the API. Developing tools in this way has a lot of advantages, however it also means you are at the mercy of third party services. Which leads me back to inspect being down. The service we were using to power the SERPs lookup we need to perform degraded over time and became unusably slow. But now we've found an even better replacement.
And now inspect is back, we're going to be adding even more awesomeness to it. By the way, if you like Inspect, why not try Scout? It's our free Chrome extension on which Inspect is based and it works superbly well.
UPDATED : This article was updated on 17/04/19 to reflect the relaunched version of Wordtracker Inspect.