There are a number of keyword discovery tools. To name a few:
- Google Keyword Planner
- Bing Keyword Planner
- Google/Bing search (related questions)
- Ahrefs Keyword Planner
- Moz's Keyword Planner
Personally, I only use Google or Bing's keyword planners. Reason being, the data is coming from the horse's mouth - the other services are making predictions, or they're just using Google's data and recycling it.