Price: Free - $25 /month
Review: Great keyword result list with tons of keywords. The option to only show longtail keywords takes hours of my research time. The "SEO rivals" feature makes it super easy to see what keywords to target. The downside is that you have to pay to use all the features, but I think it is well worth it. The niche finder is also a must try.
Price: Free - $88 /month
Review: The free version is great at giving you lots of keyword results. But there is no data on search volume. This will result in getting a lot of suggestions that might not have any traffic at all. To get search volume you have to pay the minimum of $68 /month. All in all the free version can give you a nice list to work with. The pro versions have too little value for money in my opinion.
Google Keyword planner
Review: Great research tool that is a great way to get started with keyword research. It's biggest problem is that a lot of keywords are not shown and suggestions often include lots of irellevant words. But if you have time to sort out the bad and merge results it can work great. But remember it is made for adwords and not for SEO.
Review: Is alot like keywordtool.io. Lots of keyword suggestions but no way to see which have volume or competition. But it's free so I recommend checking it out.
Price: Free - $99 /month
Review: The tool has some nice features, but at it's price point I would have expected more. There are limitations to the number of results. Wordtracker Scout is a nice chrome plugin. I recommend trying the free vesion.
Price: Free - $399 /month.
Review: The free version hides all the important data. This has very little value, but you can take the results and get more data from the Google keyword Planner.
The paid version is great. But the cheapest option is $199 per month and I would never pay that much for their service. There is a free trial, but be careful and remember to cancel if you don't wan't to pay.