There are numerous free sites where you can check out anything and everything to do with your website: load time, popularity, SEO compatibility, etc. but most of these will give you an overview of a few things, and to dig down deeper you are going to have to upgrade to a paid version.
Nibbler is no excception. For the nitty gritty you will have to upgrade, but it gives you a great overview of your status in the internet world and, best of all, you can pull up all your frenemies and compare stats in an an eassy-to-understand version.
Before I started messing with my site I think I was in the low two-millionth most popular website on the planet, but when you fiddle, something burns. And when I switched from http to https, which I hope most of you have done or are considering doing - Google likes those sites more, and if you require registration it certainly helps - I dropped to the mid eight millionth most (un)popular website in the world.
I have worked my way back up to the mid four million level, as I think Google realized my https site is the same as http, but that is still a huge drop.
Nibbler looks at your linking strategy and social media interaction and quite a few other factors, again superficially, but it does give you an idea and you can pull up your competitors' sites alongsite to see how you are faring against them. Each section is grade out of 10.
If I understood all the lingo and how to correct things myself I would consider buying a subscription, but I can imagine the glazed look in my eyes when I see the detailed analysis. I don't even really undestand Google Analytics and am looking forward to the enhanced Search Console coming soon, as I think that my be easier to work with.
I ramble. The reason I like Nibbler is because you can see how you are doing in your market area of how someone you are curious about it doing. But beware what you ask for. When you have an 8.3 in something and your competitor has a 10 obsessing could start.