SEMrush

Excellent keyword research system - can find what any site, not just our own - how and what it ranks for. Updates frequently. New features being added constantly - such as keyword difficulty. This varies drastically to other keyword difficulty software out there - such as MOZ's version...hard to know which is best ( assuming MOZ as its been around longer ) but really great software for anyone that has a website - the new features and design only adding to this.

It is not nearly as accurate as the tools that you can implement on a client's site when you have full access. Sometimes, clients will judge you based on the results of SEMRush when the results can be delayed or can ignore the long tail. It is important to let clients know that SEMRush is an estimate, and it gives you one of the best looks you can without any permission from the competitor, but it's not the best tool for measuring success on a campaign where you do have such access.

Don't take their numbers as the one and only truth. Instead, take them as a reasonable estimate of competitors and potential clients. They are a great "at a glance" tool, but they are not a good tool for in-depth analysis. There is a bit of a learning curve, but once you get the hang of it there is a ton a excellent data that can be unlocked.