Today the Iranian government has started blocking websites in anticipation for the Green Revolution anniversary this weekend. The government is worried that having access to the Internet would cause civil unrest. The Green Revolution was a movement where outraged citizens took to the streets in protest against a potentially rigged presidential election.
Although it is currently unknown which websites are banned, but we do know that some of the major social networks and e-mail services are blocked. Even websites that still frequently employ SSL/TLS for authentication (typical logins) are blocked. Some tech savvy Iranians are using workarounds by establishing network tunnels to computers outside of the country by utilizing international VPN services.