Network Information Center (now InterNIC). Xerox hired some of Engelbart's best researchers beginning in the early 1970s. In turn, in the 1970s and 1980s, Xerox's Palo Alto Research Center (PARC) played a pivotal role in object-oriented programming, graphical user interfaces (GUIs), Ethernet, PostScript, and laser printers. While Xerox marketed equipment using its technologies, for the most part its technologies flourished elsewhere. The diaspora of Xerox inventions led directly to 3Com and Adobe Systems, and indirectly to Cisco, Apple Computer, and Microsoft. Apple's Macintosh GUI was largely a result of Steve Jobs' visit to PARC and the subsequent hiring of key personnel. Cisco's impetus stemmed from the need to route a variety of protocols over Stanford University's Ethernet campus network. Internet age Commercial use of the Internet became practical and grew slowly throughout the early 1990s. In 1995, commercial use of the Internet grew substantially and the initial wave of internet startups, Amazon.com, eBay, and the predecessor to Craigslist began operations. Silicon Valley is generally considered to have been the center of the dot-com bubble, which started in the mid-1990s and collapsed after the NASDAQ stock market began to decline dramatically in April 2000. During the bubble era, real estate prices reached unprecedented levels. For a brief time, Sand Hill Road was home to the most expensive commercial real estate in the world, and the booming economy resulted in severe traffic congestion. The PayPal Mafia is sometimes credited with inspiring the re-emergence of consumer-focused Internet companies after the dot-com bust of 2001. After the dot-com crash, Silicon Valley continues to maintain its status as one of the top research and development centers in the world. A 2006 The Wall Street Journal story found that 12 of the 20 most inventive towns in America were in California, and 10 of those were in Silicon Valley. San Jose led the list with 3,867 utility patents filed in 2005, and number two was Sunnyvale, at 1,881 utility patents. Silicon Valley is also home to a significant number of "Unicorn" ventures, referring to startup companies whose valuation has exceeded $1 billion dollars. Economy The San Francisco Bay Area has the largest concentration of high-tech companies in the United States, at 387,000 high-tech jobs, of which Silicon Valley accounts for 225,300 high-tech jobs. Silicon Valley has the highest concentration of high-tech workers of any metropolitan area, with 285.9 out of every 1,000 private-sector workers. Silicon Valley