Personal relationships matter
In my experience, giving a resignation letter and two-week notice and a reference letter all paid dividends.
For one employer, I quit from the IT department, but the two weeks allowed the Finance department to make preparations to hire me as a consultant for the next two years.
For another employer, I quit from a department on good terms, and though at the time I had no inclination to return, 18 months later the same employer was hiring me back to take over for my previous boss (at a 50% raise!).
Even when leaving on bad terms, I do not see any down side to maintaining professional courtesy. At the worst you briefly humble yourself, at the best you come out a better (employable) person.
