Google's keyword tool has been one of the go-to tools in many people's SEO toolbox. For years we've taught every client how to use it as part of our normal client engagement. Now however it's gone and many people are wonder what to replace it with. I can recommend two choices:
Wordtracker has been around forever and I've been using their services for many years. They've just announced a new free keyword tool that is easy to use and returns high quality results. It does require registration.
When Google pulled the plug on the Keyword Tool, they moved most of its functionality into the Google Adwords Keyword Planner. It's focused on the Adword buyer so it is a little less friendly for general SEO work. Be prepared to spend an hour or so learning enough about it to make it useful for you. One advantage it has over the old Keyword Tool is that it allows you to drill down into more localized searches. If you are a pizza parlor in Phoenix, AZ, you are probably most interested in searches for 'pizza' in Phoenix instead of globally. The Keyword Planner allows you to do that.