Element#css_path

 doc.at("div > div:nth(1)").css_path
   #=> "div > div:nth(1)" 
 doc.at("#header").css_path
   #=> "#header" 

Element#xpath

 doc.at("div > div:nth(1)").xpath
   #=> "/div/div:eq(1)" 
 doc.at("#header").xpath
   #=> "//div[@id='header']" 

Element#swap

 doc = Hpricot("That's my <b>spoon</b>, Tyler.")
 doc.at("b").swap("<i>fork</i>")
 doc.to_html
   #=> "That's my <i>fork</i>, Tyler." 

Element#next_sibling, Element#previous_sibling

 (doc/:h3).each do |h3|
   while ele = h3.next_sibling
     ary << ele   # stuff away all the elements under the h3
   end
 end

Raise a flag for pre-release gems, both source and win32:
gem install hpricot --source code.whytheluckystiff.net.