In the world of Internet advertising, everything old seems to be new again. In the first quarter of 2006, Internet advertising surged 38% to $3.9 billion, according to a report published by the Interactive Advertising Bureau. At its current rate, online advertising should exceed the frothy levels of the tech bubble.

During the dot-com bust, the mercurial ad market shrunk, killing many New Economy companies. Even the mighty Yahoo! (NASDAQ:YHOO) had major difficulties and underwent a major restructuring, with its survival uncertain.

This time around, the situation may be less precarious. Many mainstream companies now leverage online advertising, including GM, Merck, and Procter & Gamble. These companies have huge ad budgets and incredible marketing expertise.

In addition, the difficulties of the dot-com bust brought forth key innovations in the online advertising industry. Instead of charging advertisers based on how many times an ad shows up, performance-based approaches now measure how often users actually click on or interact with ads. In Google's (NASDAQ:GOOG) pay-per-click model, the advertiser places bids to determine the prominence and frequency of ads, making the online ad market similar to the stock market.

In addition, newer technologies allow for better tracking and analysis of user behavior. Sophisticated software from companies such as WebSideStory (NASDAQ:WSSI) gives advertisers critical information on incoming traffic.

Some companies have hugely benefited from the online advertising boom. Until a few years ago, Bankrate's (NASDAQ:RATE) stock was a laggard. But current CEO Thomas Evans has made some big changes, implementing new ad models and tracking.

"The Internet has proven to be an effective medium for reaching in-market, poised-to-transact consumers," Evans told me recently. "Advertisers are able to prove that it works at an effective ROI [return on investment]."

But as the market for online advertising continues to grow, investors' biggest opportunity seems to lie in companies like 24/7 Real Media (NASDAQ:TFSM) and aQuantive (NASDAQ:AQNT). These firms provide technologies and consulting services to help midsize and large companies effectively build online advertising campaigns.

Over the past year, 24/7 Real Media and aQuantive have been strong performers. However, their stocks did fall off with the recent plunge in the markets. Assuming that the online advertising market continues to grow at the rapid clip demonstrated in the first quarter, companies like 24/7 Real Media and aQuantive should do well - especially since their shares are now trading at a discount to previous highs.

Talk tech with fellow Fools, and discover which companies David Gardner and his team have singled out for potential growth-stock greatness, with a free 30-day guest pass to Motley Fool Rule Breakers.

Fool contributor Tom Taulli does not own shares mentioned in this article. Merck is a Motley Fool Income Investor pick. The Fool has a disclosure policy.