Apple ended its Mac clone program in 1998, soon after Steve Jobs rejoined the company. At the time, there was little reason to suspect that it would ever be revived. Now a Mac clone is back, but this time without Apple’s blessing.

Psystar, a Florida-based company, is selling low-priced clones that run Apple’s OS X Leopard software, at an entry-level price far below that charged for official Apple machines. When first announced last month, many tech sites dismissed the operation as a scam, given the Web site’s proclivity to continually change its physical address and the inability of various bloggers to reach the owner.

But now the first machines have started to ship, and according to at least one well-known Mac user, they compare favorably in performance to some of Apple’s own models.

Jason D. O’Grady, creator of the mobile Web site O’Grady’s Power Page and an Apple blogger at ZDNet, recently tested a base-priced Open Computer from Psystar against its closest competitor in terms of price, the Mac Mini. He found that the Open Computer “trounced” the competition.

But take note. Like the Mini, Psystar’s $399 product comes without a monitor, keyboard or mouse. It also ships without a FireWire port, Bluetooth or a wireless card. Adding those options, plus a pre-installed copy of Leopard, boosted the price to $680, $80 more than Apple’s Mini, which comes with wireless and Bluetooth installed, but with a smaller hard drive than the Open Computer.

Because it is a clone, installing the Mac operating system is not a simple matter, which is why Mr. O’Grady had the company install it. However, according to Apple’s license agreement, Leopard can only be installed on “a single Apple-labeled computer.” That appears to put Psystar in danger of legal action. Rudy Pedraza, Psystar’s owner, said he was confident that what he was doing was legal, and Apple declined comment.

Meanwhile, it is always possible that Apple could figure out a firmware update that could disable Psystar’s clones.

But Mr. Pedraza is not worried. “We have developed technology that will allow only software updates that we have verified will work with the Open Computer,” he said.

On the other hand, if Apple decides to rewrite its software and knock out Psystar, Open Computer owners could be stuck running the current version of Leopard forever.