Traccar is a free, open-source AVL package. If your use case is primarily plotting device locations on a map and you can gather some moderate technical resources, it is a great option for you.
I like it for the following reasons:
- It has comprehensive support for a plethora of devices, including an Android (and soon iOS) client application, purpose-built trackers, and modems with embedded GPS.
- It is very easy to set up on Windows, Mac, or Linux.
- The project author is friendly and very responsive.
- It is reasonably well-architected and easy to modify, unlike its competitor OpenGTS