Packet Sender

 is an open source utility to allow sending and receiving TCP and UDP packets. It also supports TCP connections using SSL. It is available for Windows, Mac, and Linux. It is licensed GNU General Public License v2 and is free software. Packet Sender's web site says "It's designed to be very easy to use while still providing enough features for power users to do what they need.".
Uses.
Typical applications of Packet Sender include:
Packet Sender comes with a built-in TCP, UDP, and SSL server on multiple ports a user specifies. This remains running listening for packets while sending other packets.
Features.
As of version v6.2.3 Packet Sender supports the following features:
Platforms.
Packet Sender Mobile is also open source. It is available on iOS and Android and is MIT License and written in Xamarin. It only has the core features of desktop Packet Sender (send, receive, TCP, UDP, and Cloud).