Startup ideas people laughed at that eventually became very successful?

What are some of the top startups today that people thought would never work?

Most of the current large players would fall into this category.

• Twitter: It's like SMS and Email, but we'll do far less.
• Facebook: We're building another MySpace clone.
• LinkedIn: We're building a social network for professionals which they'll use once ever 4 years when they want to find a job.
• Google: We're building a search engine after all the current search engines have said search has no value.
• Mint: Give us access to all your bank accounts and show you balances with large font sizes.
• Dropbox: We're building a file syncing service even though large companies like Microsoft already have these in the market, which don't get used much.