E-mail or Snail Mail?
Today's widespread use of e-mail means that it may no longer be necessary, or even advised, to follow up through the traditional snail-mail route. There are instances, though, in which one or the other might be most preferred. For instance, if interviewing for a position in a technology-related firm, an e-mail follow-up would not only be appropriate but consistent with the company's culture and focus.
On the other hand, if interviewing with a more traditional company or with interviewers who do not seem to be technologically savvy, sending a follow-up letter through the traditional mail route may be most advised.
You can pick up cues on the organization's preference through the ways in which they have been interacting with you to date. If most of your interactions have been via e-mail, this may be most appropriate. If the company seems to favor traditional mail, that may be the best approach.
Similarly, your decision of whether to type or hand write a response will be guided by your interactions with and assessment of the climate and culture of the organization and those you've interacted with. When in doubt, you may choose to use both. Respond immediately with an e-mail follow-up and drop a more formal (or personalized, handwritten) note into the mail.
One new communication option that has not yet risen to the level of consideration here is the text message. While it may someday become an accepted form of follow-up, it is currently too informal to make a good impression.
One obvious benefit of e-mail versus snail mail is that e-mail will arrive at its intended destination instantly, whereas snail mail could take a few days to arrive. But, despite the ease and informality of e-mail, be just as formal in your e-mail correspondence as you would be in a hard-copy letter—use proper grammar and punctuation and avoid using e-mail shorthand.
Finally, don't feel like e-mail and snail mail are mutually exclusive. If you decide e-mail is appropriate, you can mention in your e-mail that you are sending along a hard copy as well. This has the added benefit of putting your name in front of the employer an extra time.