If you write varied enough topics on your website, and you rank for keywords, then you'll find some great ideas from just inside the free version of hrefs webmaster. You don't need the fancy singing and dancing one until you want to justify the cost of either that or semrush. Google adwords, google results, and free ahrefs are the 3 tools that I make use of the most. Cut through the crap, and just keep an eye on the zeitgeist of what others are doing, and track that activity with similar aspects in your own web presence, mixed with other creative thinking that you can plan and come up with. The top 20 results in many niches will have many SEO tool users, so you can piggy back off them, if you keep up with their RSS feeds, or just bookmark and check on them everyday.
I have a research spreadsheet that tracks the top 10 of the niche that I'm in, and I make analysis on that, and drop them down to fallen out when the miss the cut off point and add new ones in, but the point is, its something good to analyze. You'll be surprised by how much free information you can get that is useful if you just look for interesting data that varies between results and sites.