Arpwatch is described as 'arpwatch is a computer software tool for monitoring Address Resolution Protocol traffic on a computer network. It generates a log of observed pairing of IP addresses with MAC addresses along with a timestamp when the pairing appeared on the network' and is a network analyzer in the security & privacy category. There are six alternatives to Arpwatch, not only websites but also apps for Windows, Linux, Android, PyQt and Mac. The best alternative is ARP Monitor, which is free. Other great sites and apps similar to Arpwatch are NetCut, XArp, TuxCut and elmoCut.
Arpwatch alternatives are mainly Network Analyzers but may also be Network Monitors. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Arpwatch.