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
RTL-SDR - software defined radio
LogicMonitor - Shameless plug