Visit any forum or website to find something useful and they will ask you to register.

All websites and forums will block unregistered users, but they won’t block Google Bot. What we will do is to switch our User Agent to that of Google Bot and freely browse any website or forum without registering.  

First grab the User Agent Switcher add-on for Firefox called ‘user agent’ here and install it. 
Now go to Tools > User Agent Switcher > Options and then again to Options.


Select User Agent from the left sidebar and click Add. Now in the description field type: 

crawl-66-249-66-1.googlebot.com

and in user agent field type:

Googlebot/2.1 (+http://www.googlebot.com/bot.html)

as shown in the screenshot below.

Select Google Bot as your User Script by going to Tools > User Agent Switcher.

Now browse any website or forum without registering......