Some popular keyword tools for web developers are Wordtracker and Ubersuggest, both of which are free. Popular paid options include Raven Tools, SEMRush and MOZ.
The Google AdWords Keyword Planner is popular, but requires a paid Google AdWords account in order to use it. The Keyword Planner replaced the popular Google Keyword Tool in 2013, which was free to all and did not require an AdWords account to use.
Hubspot also offers a keyword research tool, but this is also only included with a paid account. These accounts start at $200 per month and can cost up to $2,400 per month, depending on the package selected, as of July 2015.