WhiteList

This White Listing helper will html encode all tags and strip all attributes that aren't specifically allowed.
It also strips href/src tags with invalid protocols, like javascript: especially. It does its best to counter any tricks that hackers may use, like throwing in unicode/ascii/hex values to get past the javascript: filters. Check out the extensive test suite.

<%= white_list @article.body %>

You can add or remove tags/attributes if you want to customize it a bit.

Add table tags

WhiteListHelper.tags.merge %w(table td th)

Remove tags

WhiteListHelper.tags.delete 'div'

Change allowed attributes

WhiteListHelper.attributes.merge %w(id class style)

white_list accepts a block for custom tag escaping. Shown below is the default block that white_list uses if none is given. The block is called for all bad tags, and every text node. node is an instance of HTML::Node (either HTML::Tag or HTML::Text).
bad is nil for text nodes inside good tags, or is the tag name of the bad tag.

<%= white_list(@article.body) { |node, bad| white_listed_bad_tags.include?(bad) ? nil : node.to_s.gsub(/</, '<') } %>