What does the most malicious code you ever written do?

Back in High School I created a "program" that allowed people to hack MySpace but it asked for their account info, which was later stored online. It was passed around the whole school. Once the username they were trying to hack was entered, the program sent that user a message with the app download to hack friends MySpace account, which of course would save their credentials.
It had a fake try again in 24hrs while we hack this account message if the username credentials weren't on the database.

Had about 600 credentials but I pulled the plug.
Social hacking at its best

My previous job's marketing manager had me write a program to find email addresses of people just from knowing their name and company... I felt like scum.

I wrote my own anti-software-piracy code that detected a modified program and started changing random bits of code every five minutes

a set of backdoors in websites I made for a certain client, just in case they didn't want to pay the service.
I must say that that the backdoors did work very well!
I didn't got the money, but they didn't got the code either lol
+ I had the fun to watch it burn