Other Internet things that I think are cool:

pwnat - communicate with servers behind a NAT w/o port forwarding or DMZ

dump1090 - Decode ADS-B/Mode S from aircraft via RTLSDR

ruby koans - learn Ruby with the Edgecase Ruby Koans

pass - The standard unix password manager

sshuttle - Quick and dirty VPN with ssh, no admin required

OpenBSD - Because you’re too good for Linux too.

arp-scan - Scan your network with ARP.

ddrescue - Recover data from a busted block device.

Beej’s Guide to Network Programming - Great guide to socket programming in C