Glad someone mentioned HMS Bulldog and the capture of the Enigma codes from U110. Undoubtedly of huge influence, albeit temporarily as code books were changed over time.
Just to throw a completely different idea out there - how about the German auxiliary raider Atlantis? In Nov 40 she captured the British ship Automedon which carried the complete order of battle for the British in the Far East and showed just how weak the defences in Singapore actually were.
Germany duly passed that intelligence to Tokyo and it undoubtedly had a huge impact on Japan's decision to embark on the southern strategy once the American trade embargo against Japan started to bite in July 1941. With British weakeness in the Far East clearly identified, Japan had confidence in the ability to embark on their aggression against French Indochina, Siam, Malaya, Singapore, Burma and the Dutch East Indies.
In addition, with the southern strategy identified as the main axis of effort for Japan, the need to limit the influence of America's Pacific Fleet and protect the route from Japan to the Gulf of Thailand led to the attack on Pearl Harbor and the invasion of the Philippines.
Overall, quite an impact methinks!