"Harry Potter asks if he can help Dobby ... Dobby has heard of your greatness, sir, but of your goodness, Dobby never knew."

He enchanted a Bludger,

• closed the magic portal that leads to platform 9 ¾,
• interfered with Owl Post and stole letters, and
• knocked Lucius Malfoy down a flight of stairs.
• He also can disappear at will, even at Hogwarts where Apparition is "simply not possible."
• Dobby claims to be a secret keeper for Dumbledore.
• Apparated in and out of an "escape proof" cellar at the Malfoy mansion.

Note that as long as Dobby was the slave of the Malfoys, his attempts to help Harry failed:

• the stolen Owl Post
  
• the closed magic portal leading to Platform 9 ¾
  
• the enchanted Bludger
  

But after Harry Potter freed Dobby from the Malfoys, Dobby

• knocked Lucius Malfoy down a flight of stairs
  
• gave Harry Gillyweed so he could breathe underwater and rescue Ron 
• he explained how and where Harry could find the Room of Requirement 
• and warned the members of the DA when Umbridge discovered their secret
  
• tailed Draco 
• Apparated into the Malfoy Mansion on a rescue mission from Aberforth
  
• Rescued Luna, Dean Thomas and Ollivander from an "escape-proof" cellar 
• Dropped a chandelier on Bellatrix
  
• Stripped Narcissa of her wand
• Rescued Griphook, Harry, Ron, and Hermione from certain death 

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