Log Parser is a free and unsupported command-line tool popularized by Mike Gunderloy and developed by Gabriele Giuseppini of Microsoft.  Log Parser enables you to run SQL queries against all sorts of log files and system data sources.  You can return the results in all sorts of target formats, such as SQL tables or CSV files.

If Microsoft wants to do more things like the open-source community does, this is a good example of success.  The website, at http://www.logparser.com has a Forum, Knowledge Base, Resources, and Repository.  The Knowledge Base looks very familiar if you're used to Microsoft Q articles.  And the Repository has lots of useful scripts, such as a script that calls the Log Parser using C# and another that measures the total traffic over a given period of time for one or more IIS sites.
 
You can get Log Parser 2.1 (the current version) by downloading it from the Resources link on the site and installing theIIS 6.0 Resource Kit Tools, but you don't need IIS to use it.

For those of you who don't like to spend a lot of time learning tools, no matter how useful they are.  There's an excellent recorded webcast that covers Log Parser basics, tips and tricks, and some previews of upcoming features in version 2.2 at TechNet Webcast: IIS Data Mining with Log Parser 2.X.
In addition, check out Mike's Weblog at http://www.adtmag.com/devcentral.asp.

Let me know if you found it as useful as I did.  I'd also like to hear about any useful or creative ways you've used the tool.