After years of watching and wondering, the day analysts and investors have been waiting for has finally come: Facebook announced today that it plans to file for its initial public offering (IPO) in April. The world's largest social network said it plans to sell off $10 billion worth of stock which would give it a hefty valuation of $100 billion, making it the biggest tech IPO in history. It would also make Mark Zuckerberg incredibly wealthy. But what other companies have caked off with huge IPOs? Read on to find out.