Initially a side-project and an excuse for getting my hands dirty with AWS serverless technology, is now being used by a number of open-source projects; specifically pentest and red-team tools. The benefit of dynamically grabbing the latest User-Agent headers is two fold:

The first is that you don't have to manually keep a list of User-Agent headers updated every few weeks/months.
The second, arguably more important reason is that detection of red-team/pentest tools will be made slightly more difficult as the User-Agent headers sent by these tools will not stand out (the number of tools I've seen with User-Agents still sending single digit Chrome and Firefox release numbers is staggering)

API Usage

The API is served by AWS API Gateway and the User-Agent header is constructed by AWS Lambda.
User-Agent headers are updated every 6 hours.
The following browsers are supported: chrome, firefox
The following operating systems are supported: windows, mac/osx
This will give you the latest version of Chrome on a Windows OS
This will give you the latest version of Firefox for OSX