Heap

• No need to predefine tracking events - 'Autocapture' automatically captures every user interaction from web, mobile, and cloud services.
• 'Virtual Events' let you retroactively manage user interactions without the need to change any code or alter the underlying data.
• Behavioural analytics (updated in real-time) allow you to analyse events, segments and properties.

Heap offer a free plan with 5k sessions per month, increased to 50k if you add their logo to your site. Paid plans are based on session volume, and there are also additional add-on features that can be purchased.

You'll need to include your Heap tracking snippet in your website or mobile app, and in most cases you will probably also want to enrich your data with user information using their Identify API.