The Keyword Niche Finder returns groups of keywords instead of just a long list of keywords related to the term. This is a good place to start if you haven't already done keyword research for the campaign you're working on.
For example, let's enter "Christmas shirt" into the tool. The initial results return various groups:
Click on one of the groups to view the keywords and their frequency:
The only thing I don't like is the inability to access the full list of niches. You would have to delete existing groups in order for groups down the list to appear. You can have the groups emailed to you, so theoretically, you could access all the groups by deletion. But that's very time consuming.
I guess this is to be expected with a free tool that's designed to get you to sign up for a premium product.
If you already have a list of keywords, the Keyword Grouper tool will group them for you. Simply copy and paste your list (and frequency if you have it) into the Grouper tool.
Then you get results grouped, just like in the Keyword Niche Finder.
Both tools work extremely fast and are quite decent for being free. If you're on a super-tight budget, you can use this in conjunction with the Google AdWords keyword tool. The auto-grouping will save you a little bit of time.