I have to use Apache Tomcat to test a legacy site, prior to upgrading it. I just grabbed the most recent release of Tomcat, unarchived it and ran it, and got:
This release of Apache Tomcat was packaged to run on J2SE 5.0
or later. It can be run on earlier JVMs by downloading and
installing a compatibility package from the Apache Tomcat
binary download page.
Must. Resist. Urge. To. Rant. About. Tomcat.
Sometimes I really appreciate how communities such as Cocoon focus heavily on supporting older JDKs by default.
Update: From the README:
Tomcat 5.5 requires JRE 5.0 by default. Read the RELEASE-NOTES and the RUNNING.txt file in the distribution for more details.
From the release notes:
Tomcat 5.5 is designed to run on J2SE 5.0 and later, and requires configuration to run on J2SE 1.4. Make sure to read the “RUNNING.txt” file in this directory if you are using J2SE 1.4.
So I suppose I can’t complain, though I think this sort of information might be more useful on the page listing downloads – who actually reads README files? 
Related posts:
Perhaps this is a mistaken belief, but I always thought that recent Tomcat versions required Java 5 because the version of the servlet spec it implements requires the Java 5 APIs underneath.
I definitely could be wrong though.