Apple's market cap is so high that only three publicly-traded US companies have market caps higher than Apple's: Exxon, Microsoft, and a little retailer called Wal-Mart. Apple will probably never catch up to Exxon's market cap, which exceeds Apple's by more than $100 billion, and it has another $55 billion to go before it can surpass Microsoft -- not likely, but not impossible. Apple's market cap is within striking distance of Wal-Mart's, however, with only about $5.5 billion more to go before Apple surpasses the market value of the world's largest retailer.
What's even more interesting than the companies still above Apple are all the companies below it. Apple's market cap exceeds Google's by more than $30 billion, and 1980s rival IBM is nearly $40 billion below Apple's value. Even more telling than that: in terms of market capitalization, Apple is now worth more than Nokia, RIM, Sony, Motorola, Palm, Dell, and Adobe -- combined.
Apple's all-time low market value was back in 1982, when it was a mere $640 million. For perspective, that's more than $37 million below Palm's current market value (before adjusting for inflation, naturally), and Palm is widely expected to go belly-up any day now. 28 years later, Apple has grown to occupy a position in the market that I doubt anyone would have dreamed of even only five years ago. Whatever else you might think of Apple, one thing is undeniable: someone at the company is doing something right, because they're raking in money like almost no other corporation anywhere.
Disclosure: I own absolutely no Apple stock whatsoever, but I wish I could tell the 2004 version of me to go buy some so I could retire next year.