SAN FRANCISCO (CNN/Money) - From the looks of the penguin paraphernalia fluttering around San Francisco's South of Market district these days, one might think a Batman fan club was holding court at the Moscone Convention Center.

But the fliers, posters, and costumed waddlers are actually part of the annual LinuxWorld convention, which explains why the penguin -- the universal mascot of all things Linux -- is everywhere.

Missing from the swag are Linux-related stock certificates, however, many of which, like pennies on the promenade, are hardly worth bending down to pick up these days. The market has soured on most things Linux.

Pure-play companies such as Red Hat (RHAT: Research, Estimates) and VA Software (LNUX: Research, Estimates) (which changed its name from VA Linux but still trades under the ticker LNUX) are charter members of the "90-percent-plus club," made up of stocks that are down more than 90 percent from their boom-era peaks.

But I'm taking potshots here, and for that I'm sorry. Red Hat is actually the market leader in Linux programs, having inked partnership deals with Dell, Hewlett-Packard, IBM, and others. And beneath the stock market detritus, the technology behind these Linux companies -- if not the companies themselves -- is providing tremendous value to corporations and, hence, their investors.

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Research firm IDC estimates that spending on Linux operating systems will increase over the next five years, from $80 million in 2001 to $280 million in 2006 -- a 28 percent compounded annual growth rate. IDC also predicts that Linux server shipments will increase to 622,000 this year, up 28 percent from 2001.

During the same time period, Windows server sales are expected to increase 12 percent to 2.8 million units, even as Unix server sales jump 5 percent to 693,000. Numbers like these are a great relief to an industry that has been looking down and to the right.

Tech giants are lining up with new products for this steadily growing market. Oracle announced that it is releasing its open-source-based Cluster File System, aimed at making its database software run better on Linux systems. Sun Microsystems also joined the party, unveiling its first general-purpose Linux server earlier this week.

The Sun announcement "is one of the most important things we're going to see this year," IDC analyst Al Gillen says. "Is it something IBM can't offer? Not really. But it will be perceived as being different enough that customers will look at it and consider it."

Companies such as Dell, HP, and IBM have long touted the benefits of Linux software and open-source programs. Just in time for the LinuxWorld convention, IBM announced a partnership with VA Software: VA's open-source project collaboration program, SourceForge, will run on IBM's DB2 and WebSphere products.

Missing from the product announcements -- but not the party -- is Microsoft. Mister Softee is bravely manning its first-ever booth at the LinuxWorld convention, but the company hasn't made any Linux announcements, and no one expects it to. At the moment, Microsoft is simply trying to avoid alienating itself from the growing ranks of Linux developers in the marketplace.

Linux devotees have morphed from a famously shaggy bunch to solid citizens who work at buttoned-down companies like IBM. Tech investors should watch carefully to see which companies adopt Linux and which do not. For investors, the penguin means the possibility of growth, something sorely lacking in much of today's technology landscape.

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