I see a lot of people are asking for it, so here is a legal and simple way to do it:
Go to:
http://www.pagereboot.com
Once the page opens, type the complete address of the website you want to target and set seconds to 1. Click start. Leave the page running as long as you can. The website will reload every 1 second, and if enough people do this it will eventually lead to the crash of the server.
You can do this for multiple sites at the same time (just open a new browser or new tab).
Here are some websites I recommend you target:
Supreme Leaders Website:
http://www.khamenei.ir/
Press TV- Government Propaganda TV
http://www.presstv.ir/
IRNA: Official News Agency of Iran
http://www.irna.ir
Ahmedinejad
www.President.ir
Foreign Ministry:
http://www.mfa.gov.ir/
Ministry of Interior:
http://www.moi.ir/
Police:
http://www.police.ir
Justice:
http://www.justice.ir
Lets at least make it harder for them to publish their propaganda!
EDIT: Flyswatterbanjo points out that the Iranian government is attacking websites run by expats who are opposed to the regime:
"TehranBureau problem w/ site pinpointed: webmaster says the Iranian govt is overloading us with requests to disable our site: "denial of service attack"
half a minute ago from web"
These people will stop at nothing. They are trying to hide this massive fraud they have committed on Iranian people.