There are many ways to contribute to opensource software (OSS), programming only being one of them. I develop OSS, but use OSS too. For example, I am a big user of the Linux kernel, the KDE desktop, Kubuntu, Debian (I have unstable in a chroot), Firefox, Eclipse, Classpath, and many, many others. What these have in common, is that I generally have no time to look into the source code of these projects. A small patch excluded, I am really a regular user of these projects.
However, I try not to leech (see also Peter's related comment on that): I care about these projects and, therefore, I file bug reports. Sometimes, I even join the developers and talk to them via commonly used IRC and mailing lists. Even, every now and then I get this itch and then I do look up source code and contribute a patch. But filing bug reports is the least one can do, the least everyone should do.
Classpath is the GNU project to provide a free Java library, i.e. the set of java.* classes that come with the Sun JVM. It is not a virtual machine, though, for which several opensource implementations are available, many of which use Classpath as library provider. They have a very nice chat channel at irc.freenode.net, called #classpath. There wiki provides a platform for given feedback on how well software runs. A bug track system (BTS) is available too. An overview of the bugs that I filed, can be found at my del.icio.us account: bugreports+Classpath.
Needless to say, Classpath is important in making our Java based chemoinformatics truely opensource.
Things are different for Debian and Kubuntu: these are distributions and, except for some patching, are generally not involved software development as done by upstream. However, they generally do appreciate to know about bugs too, so there is some duplication of bug reports here.
That said, they do provide nice tools for bug reporting which works for all packages that they distribute. Debian has reportbug and Kubuntu has Launchpad. An over view of bugs I reported with Debian can be found at del.icio.us bugreports+debian. I do not have bug reports in Launchpad yet, but two can be found in mailing list archives, see del.icio.us bugreports+ubuntu.
I also tracked back two bugs I reported with KDE, see del.icio.us bugreports+KDE.
Surely, I filed many more bugs to many other projects. A long list of bug reports can be found on SourceForge. However, it seems not possible to make an easy list of that :(