ipconfig
Displays network information
shutdown -a
Cancels a shutdown, you can do this from the run line though. So if some ******** clicks the button you shutdown the computer you can quickly type that in to stop it.
shutdown -s
Shutsdown the PC
shutdown -s -t 60
Shutdowns the computer in 60 seconds. The time is configurable though. I usually do this if I want to watch movies or TV shows from my computer as I go to sleep, so I'll set the time to 3600 seconds (1 hour). Also can be ran from the Run line.
dir (filepath)
It's a file directory.
telnet
Useful for remote assistance, connecting to routers/switches, or some other neat stuff. Try typing "telnet avatar.outland.org 3000" into command prompt. It's a command prompt game. Another cool one is "telnet towel.blinkenlights.nl". It's a recreation of Star Wars using text in command prompt.
ping
Good to find out the IP address of a website (e.g. ping yahoo.com). It can verify your connection to the internet and to your router.
tracert
Can help identify a routing issue in a network.
color 0a
Changes text in command prompt to green, so it's like the matrix yo. If you want to change the background color or the foreground color you can type color help and see which other combinations you like.
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There's a few other ones, but those are the cool ones.