The "Department of Injustice" attack was arguably the most severe. The departmental seal was replaced with a Nazi swastika, and Adolf Hitler was pictured as the Attorney General. A close-up picture of female genitalia underscored, and undermined, the vandals' stated political motivation: opposition to censorship on the Internet.
The CIA, by contrast, got off lightly. A group calling itself "Power Through Resistance"changed the banner headline to "Welcome to the Central Stupidity Agency", made a libellous attack on an individual, and changed the links to other pages.
Unpopular government departments are not the hackers' only targets. Companies have data that can be sold to competitors, and payments systems that can be subverted to issue cheques to crooks. Some British firms, according to rumours on the Net, have already been hit, but understandably, no-one will confirm the attacks. In all, though, hackers and viruses caused an estimated $60bn of losses and damage worldwide last year.
As the Web rolls on as an advertising medium, so too does the potential damage that vandals, motivated by ideology or some perceived grievance, can wreak.
The Web site attacks illustrate not only the vulnerability of modern computer systems but the speed with which hacking is evolving. Until a year ago most security concerns were focused on hackers getting access to confidential, even top-secret data, or the infiltration of viruses that would flash unwanted messages on screens, if benign, or crash hard disks if more vicious.
While these threats have not lessened, others have been added. E-mail, new Java language programmes - which travel via the Web to computers - and even sound and video clips are vexing security chiefs. Soon encryption systems for credit card and electronic cash transactions will join the queue as potential targets.
"We have to come up with a new application almost every month," says Dr Dev Triant, chief executive of the marketing arm of Checkpoint, the world's largest computer security firm and the second largest Internet stock, after Netscape, on the New York's NASDAQ exchange.
Checkpoint, an Israeli company now worth $1bn, achieved this dominance through the design of the first "firewall" programme that did not require extra hardware to be added to a computer system. Firewalls are the programs that stop unwanted access from the Net and, with 15,000 installations, Checkpoint now has 40 per cent of a $500m world market that is tripling in size each year.
Now, like IBM or Bill Gates' Microsoft, it is setting the pace in trying to fix common standards for the industry worldwide.
Like most security companies, it is reluctant to claim invulnerability. That would be tempting fate. But its own technicians have been unable to crack its walls, even with their detailed inside knowledge.
It also survived a concerted assault by the cream of the hacker community. 60 Minutes, an American investigative journalism programme on the CBS network, invited two hackers from a known New York hang-out for computer cowboys to its studios to test the company's defences. Word spread, however, across the world within minutes.
As a result, Checkpoint's unsuspecting technicians in Tel Aviv recorded an unprecedented 30,000 attempts to break in over a 24-hour period. All failed.
It is an impressive record, but Checkpoint is the first to admit that its product range is limited. Bright sparks in the cyber world continually add useful new doorways to computer systems for video, E-mail and other data exchange, and no one company can guard them all.
Checkpoint's firewalls, for example, will stop outside users from signing on to a system without a valid password, while allowing them to see information - like Web sites - that the provider wants to make publicly available. It cannot, however, stop an E-mail message from carrying a subversive program into a system.
Like Bill Gates before, Checkpoint is releasing data on how its programs are structured so that other companies can design products that will fit like modules alongside its own firewall.
But it is far from universally accepted and it is meeting some resistance. One rival said the standard should have been agreed by all rather than dictated by the leading supplier.
There are also concerns that the release of data could actually help hackers. One London-based consultant with a major US security company also said that, while it will probably speed up the adoption of security programs, it will be mean that any weakness will become standard too. "One big difference between muggers and hackers is that the hackers have to be smart," he said. "If there's a way in, they'll eventually find it."
Dr Triant, a mathematician by training, believes the more serious threat comes not from computer cowboys sitting at home surrounded by ashtrays and Coke cans, but from disgruntled employees. "Some 90 per cent of security breaches can be traced back to employees," she said.
If an organisation's internal security over simple things such as passwords is lax, it will always be at risk from the hackers.