Here's an interesting little app that's useful for annoying your friends. WiFiKill is an Android application that lets you DoS the whole WiFi network your phone is connected to, or kick specific devices off the network. Obviously this is something you don't want to do in any place where Internet connectivity is important, like your workplace.
I do think it's a fun little gag for when you're with friends or family at a residence though. One minute everyone is on their computers checking email and Reddit, the next they're all getting page load errors and you're laughing like a maniac. And you can do this with just your phone, without even needing to understand how it works.
It's spoofing ARP replies to make devices on the network think your phone is the router, and then it drops the packets from those devices that you've decided to deny service to. The person on the other device is still connected to the WiFi network, but nothing in their browser or other net connected applications will load for them.
You can defend against this sort of ARP spoofing on your own network by setting up a static ARP table, read this for more on that. You can detect if someone on your network is running this sort of attack on you by running Wireshark and checking if you're recieving a lot of ARP packets without sending any requests, or if the router MAC address has been changed from what you know it is.
You'll need root on your phone, so check out the XDA forums if you need help doing that. You'll have to head to XDA anyways to download WiFiKill, since Google pulled it from the Market.
Don't pull this trick at work. Your sysadmin will hunt you down and an angry sysadmin is a fearsome thing. This is just a neat way to mess around with your friends once or twice. Don't overdo it.