Keyword Spy is an online app (or website) that helps marketers and SEO professionals track the performance of their website, keywords, and competitors. It provides an in-depth analysis of the online landscape, including analysis of competitors’ keywords and their performance, historical data, and keyword trends. It also provides insights into how and why certain keywords outperform others, and it helps users to identify new opportunities and threats in their niche. With Keyword Spy, users can monitor their own performance, track competitors’ keywords, and gain insights into their own SEO strategy.
Serpstat is much better and cheaper. Also, I'm a big fan of Serpstat's Tree View feature. It's useful to optimize and then crawl pages to the first SERP page.