Kimai
Kimai is an open-source time-tracking solution from Austria. With users grouped in teams, customers and projects can be assigned to those teams; invoices for your customers can be generated directly from their clock-in and clock-out data; and reports can be generated to gather insights about your workforce. In addition, they have an API, so you can integrate their service into your infrastructure or write your own plugins.
Pricing
Their hosted standard plan costs EUR 2.99 monthly per user.
Read more on the pricing page of the service.