I have a simple socket server, that waits for a message and responds. How can I test this? Is there a client app or something that can help me send some data on a specific port to a server, and see the output? Telnet?
Simple telnet client works well for such tests. You can also try PuTTY in either Telnet or Raw connection modes. Both allow you to choose the port you want to connect to.
Also a tool like Microsoft Network Monitor is pretty useful to analyse the protocol's data flow if you don't have direct control (by logs) on what's being sent over the wire.