Thank you note after interview

What do you usually do for your thank you notes after an interview? Do you type a business type letter and send it? A handwritten note? A Thank-you card with handwritten note? Or do you send an e-mail?

I feel a letter is the best way but in these days, an e-mail would get there faster. And if there are a few people up for a job that might make a difference, right? But is an e-mail less formal and appear to take less effort?