Without register you can browse forum

most of the websites will not allow unregistered users or guests to enter, but they do allow “Google Bot” or any other search engine bot to enter for crawling purposes

You will need to change user agent to Google bot for browsing forums.
What you need is Firefox Browser and User Agent Switcher (Serch On Google) After installing, create another user agent from the options menu of the addon.
Select User Agent from the left sidebar and click Add. Now in the description field type:
Code:
crawl-66-249-66-1.googlebot.com
and in user agent field type:
Code:
Googlebot/2.1 (+http://www.googlebot.com/bot.html)
Now Start browsing by selecting the user agent as Google Bot. Please check out
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Good job brother!