keyassort.com
When the question of keyword clustering comes up, nothing better than keyassort.com comes to mind.
Out of 2,900 keywords with a clustering threshold of 3, 500 groups remain, which is very convenient for avoiding duplicates.
Advantages Compared to Others
— One-time price of $30
— Low cost for scraping SERPs — essentially you only pay for xmlriver.com limits, which pulls data from Google for something like $0.20 per 1,000 keywords — pennies…
— Fast speed — nothing is faster than desktop apps, both for position tracking and clustering.
Semparser.ru
A decent SaaS service that I also use, but the cost is non-comparable to Keyassort.