Sometimes it might be useful, but mostly just looking cool or impressive to visualize log files (anything from http requests and to bandwith usage to cups of coffee drunk per day). I know about Visitorville which I think look a bit silly, and then there's gltail.
How do you "visualize" your log files in realtime?
Using SciTe, Notepad++ or other powerful text editor which have file processing routines, so you can create a script that colorizes parts of the log or just delete some non-important lines from it
You may take a look at Apache Chainsaw. This nifty tool allows Log incomes from nearly everyqhere and has live filtering and colering. If you have an already written Log, I'm not sure if it can read it, it's been a while since I used it last time (was very usefull for the prototyping phase of our JBoss server)
Google has released the Visualization API that is probably flexible enough to help you:
The Google Visualization API lets you access multiple sources of structured data that you can display, choosing from a large selection of visualizations. The Google Visualization API also provides a platform that can be used to create, share and reuse visualizations written by the developer community at large.
It requires some Javascript knowledge and includes Google Docs integration, Spreadsheet integration. Check out the Gallery for some examples.
We use Awk and Perl scripts to parse the log files and create summary reports and "databases" (technically databases in that each row corresponds to a unique event with many columns of data about that event, but not stored in a traditional database format. We're moving in that direction). I like Awk because you can very quickly search for specific strings in the log files using regex, keep counters and gather data from the log file entries, and do all kinds of calculations with that data. Then use your favorite plotting software. We use Excel, mainly because that's what was here before I started this job. I prefer MATLAB and it's open-source cousin, Octave, which is built on gnuplot.
There is also the logstalgia tool. Visualizes Apache logs. See http://code.google.com/p/logstalgia/ for more details and a youtube video.
I prefer Sawmill for visualizing data. You can basically throw any log file against it, and it will not only autodetect its structure*, but will also decide on how to analyze it. Even if you have a custom log file, you can still define what and how shall be analyzed and visualized.