All these tools are being run under Linux.
Network Manager is a desktop program that’s installed with most distributions and allows you to alter network configurations from userland. It has a feature where you can create multiple connection profiles, so you can have different profiles for different IP address ranges. It’s useful when you reset a device and need to connect to it via the default IP address. It’s also useful when you want to renew your DHCP lease – just click on the connection and Network Manager will reset the interface.
Wireshark is an ethernet packet analyzer, and is useful when you want to see what’s going on when the network lights flash. It’s useful for checking that broadcast packets don’t cross VLANs.
Ping is ping – a classic program, and “ping -b” is useful for sending broadcast packets.
netcat opens connections between computers using TCP or UDP. Useful for testing bandwidth limiting.
ifconfig shows you your interfaces and their assigned IP addresses.
nslookup or dig, to test DNS.
ab, Apache Benchmark, does load testing for websites, and is also a handy tool for generating http traffic on port 80.
Asterisk is a VOIP system, and can generate SIP traffic. I’m an asterisk newbie, and don’t know the traffic generation tools out there.