Internet advertising is booming. The industry has gone from $9.6 billion in revenue in 2001 to $27 billion this year, according to Piper Jaffray, an investment bank. And it is still early days. The internet accounts for only 5% of total spending on advertising, but that figure is expected to reach at least 20% in the next few years. The single largest category within this flourishing industry, accounting for nearly half of all spending, is “pay-per-click” advertising, which is used by firms both large and small to promote their wares.