Can I register a unique website address that is a twist on the name of a well-known company?
Trademarks are intended to prevent consumer confusion. If you register a domain name that is intentionally designed to confuse consumers searching online, then there is a good chance that you will find yourself in litigation with the well-known company whose trademark you have modified.
Even if you are not directly infringing on the company's trademark, you might be diluting that trademark. Trademark dilution is the use of a mark, or similar mark, in commerce in a manner that reduces the value or recognition of the original mark in the public's perception.
So, for example, if you were to create a website called "www.FalMart.com"—which sounds like WalMart—you might be diluting WalMart's trademark. This sort of use blurs the correct name in the public's mind, and also tarnishes the good name of WalMart.
Generally, large companies have in-house lawyers who will aggressively police their trademarks by constantly searching the Web for infringers. It is likely unwise for you to bait a big company into litigation, particularly since there is probably little commercial advantage you will receive from your actions and a high likelihood that the company will enforce its rights in a costly lawsuit.