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 (well did work) fantastically well at uncovering closely semantically linked keywords that other tools just could not see.
However right now it's broken. 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. You'll notice the SERPs is currently missing from the keyword tool as well.
Within Inspect when the SERPs took too long to return it was causing a further issue with pending results timing out and the tool just stopped working. We're working on either finding a better solution for SERPs results or building a service ourself if we can't. Once we have something Inspect will be back up and running.
In the meantime, why not try Scout? It's our free Chrome extension on which Inspect is based and it works superbly well.