Many famous artists such as Clapton, Steve Vai and Joe Satriani have their own special guitars and are usually endorsed by guitar manufacturers. From time to time these manufacturers offer limited edition copies of the stars guitars which by their very nature are highly desirable.
As the number of collectable guitars produced is very low, they become much sought after and are looked at as a long term investment. Some collectors buy the guitars and never play them, while others buy them for regular use every day. Most manufacturers also offer anniversary models, such as Fender with their 50th and 60th anniversary Strats and Ibanez with the 25th anniversary RG guitar. These instruments are only available in limited numbers and worldwide rarely exceed a couple of hundred in total.
Recently, collections of "art" guitars have also been launched by various manufacturers such as Fender, Gretsch and Martin. A limited edition guitar will never be at the budget end of the market, but then again will never lose its value!