I've always found the idea of intelligent every-man sent back in time using his knowledge of the future tech to realistically impact the past. The first time something like this came to my mind was in computer science class when it finally struck me just how drastically our computer hardware has increased, and how impressive it would be to go back to the beginning of the computer age with my laptop and knowledge of all our programming history to shape it.
One of the obvious things to do with a story like this would be to drop someone into the World War II and Cold War eras. Let's say I suddenly showed up right at the beginning of World War II, and somehow managed to show my laptop to someone important enough to get funded to support the war effort. First, how hard would it be to write a program to brute force break Enigma, or similar a scheme they may switch to once they realize Enigma was broken?
I know how to brute force break a simple cypher, but I don't know Enigma's approach as well. As I understand there were 3-4 dials they could switch from, so if I simply iterated through every option for each 'dial' till I get a valid looking message would that be sufficient, or was Enigma more complex then that? I would need to get a Japanese and German dictionary in my computer, if I don't have one already hiding out in Word somewhere presumably some typists would be required to trade off typing duties for a week to create the dictionary file.
Once an Enigma brute force approach was written what would happen? Let's assume that the presence of the laptop was kept secret from the enemies. Would the Allies abuse their cracking system excessively on the grounds that even if the Axis changes its encryption a new brute force solver could be hammered out within a week or so of scripting? Or would figuring out how something was encrypted to know how to brute force it be a sufficient enough challenge that we would still try to not reveal we had broken them?
What would it mean to the war effort to have Axis communication broken from day one? How significant a change to the war as a whole would occur?
For the sake of an interesting story, that doesn't make the time traveler a god, assume that the time traveler had not expected to travel in time and always hated history, and as such has a very limited knowledge that could be useful. Perhaps he knows a bit of Pearl Harbor, D-day, Midway, and the drop of Atomic Bombs, but nothing extensive and he is horrible with exact dates (no saying when Pearl Harbor happened, only that it did).
Finally, what other immediate or significant effects could such a travel have on the war? Assume a cheap laptop and basic (no more than newbie college graduate) programming skill, a general interest in the history of computer development and internet particularly, and a decent knowledge of science/physics and it's history as well. However, assume that the protagonist is not a hardware specialist and has only limited knowledge of computer architecture and hardware except for when it impacts software development and performance (no giving hints on how to build better computers faster).