Various sites are blocked by authorities with filters on DNS (Domain Name Server), to bypass them you just need to change them and use public DNS, Google and OpenDNS offers this service for free.
You just need to manually modify the addresses in the properties of TCP/IP v4 of your connection (both LAN and Wireless).
If you want to use Google DNS, the settings are:
or you could also use OpenDNS:
If you don’t want manually setup DNS you can use Public DNS Server Tool, a small free utility.
[UPDATE 03 April 2018]
There is a new DNS service by Cloudflare, popular company that provides networking and internet security services.
You just have to use the following IPs: