You may not have heard of MySQL, but I can almost guarantee you've used its database at some point. If you've ever filled out a form on the Web or purchased something online, there's a good chance the data you provided is stored in a MySQL open-source database. This simple-to-use database is behind many of the transactions that take place on the Internet, because unlike Microsoft's (NASDAQ:MSFT) database products, it's free and it runs on both Linux and Microsoft operating systems.

I've used MySQL many times, and the database does live up to its reputation for being easy to use. I've also worked with many corporate databases and can tell you that MySQL is more than adequate for handling the majority of applications databases are used for.

Information-intensive companies with large and long-standing databases hooked into mainframes and massive servers want to stick with the ultra-heavy-duty databases supplied by IBM (NYSE:IBM) and Oracle (NASDAQ:ORCL). But for many of the new databases being built and departmental-focused setups, MySQL is adequate -- save for a few rare cases.

Many companies are quickly coming to realize this, as you can see from the impressive customer list on MySQL's website. I'm sure that most readers have used the Yahoo! (NASDAQ:YHOO) Finance site on many occasions (like perhaps this one), but would probably be surprised to learn that even Yahoo!'s massive source of data is fed from MySQL databases.

MySQL and a few other lesser-known open-source databases are competing directly with Microsoft's SQL Server, though it isn't apt to have a big impact on Microsoft's bottom line because it's such a small part of the company. Even though open source is taking far less market share from it, Oracle is the company getting hit the hardest, because it is the least diversified. And that is at least, in part, why the company has been so desperate to acquirePeopleSoft (NASDAQ:PSFT), which offers highly specialized complex database and application combinations.

While Oracle and IBM can maintain most of their big long-standing customers, there's no doubt that they're losing new ones to MySQL. And Microsoft might very well find its database products nearly obsolete in a few years if MySQL usage continues to grow 30% a year, as it did in 2003.

Do you think MySQL is a threat to Oracle? Share your thoughts with other Fools on the Oracle discussion board.

Fool contributor Mark Mahorney doesn't own shares of any companies mentioned.