FileZilla (for Windows and Linux). This is the best FTP/SFTP program I’ve ever used. It reminds me a lot of CuteFTP from when I used it in the late 90s.
PuTTY (for Windows). A fantastic Telnet and SSH client for Windows. It’s simple, but has every option I could ask for.
Wireshark (for Windows and Linux). An awesome packet capture program. It used to be called Ethereal1.
1 Technically it’s a fork of Ethereal. I think the lead Ethereal developer left the company he was working for, but that company owned the trademark to the name Ethereal, and I guess they weren’t happy about that so they forked the project and all the Ethereal developers switched to working on Wireshark.