After doing some nosing around I found Princeton University's lexical database that offers suggestions for related words. It works, but isn't good with phrases, or colloquial terms.

Then I remembered that Google is pretty good at giving related word examples, not directly though. By using the tilde ~ synonym finder operator in a search you'll get shown related results, and more importantly the SERPs will show you whether you are pursuing the right keyword if it has multiple meanings.

So, if you're struggling to cover as many bases as possible in your initial keyword research a simple ~ can go a long way. For instance, if you've got a website that sells road traffic monitoring equipment, and you've got robust version for a building site you'd probably want to know if "site traffic" is going to be of much use to you. By searching with ~site traffic, Google shows what it reckons:

Here we can see that although the AA and Highways Agency get a look-in the majority of results relate to website traffic, so it wouldn't be a good option.