boinc_samples (check this out in the same parent directory as boinc, and the relative paths will work). The example applications are:

upper_case: a full-featured example BOINC application. The application does things (like checkpointing and graphics) that can be tricky or confusing. You can use it as a template for your own BOINC application; just rip out the computation part (which is trivial) and replace it with your code.
The graphics show a bouncing 3D ball. If you put files Helvetica.txf and logo.jpg in the directory where it runs, you'll also see an image and some nice-looking text (thanks to Tolu Aina for the latter).
wrapper: used to support legacy applications.
worker: a representative 'legacy' application (i.e. it doesn't use the BOINC API or runtime library). Used for testing wrapper.
sleeper: test application for non-CPU-intensive projects (used for testing the BOINC core client).

The boinc_samples tree includes project files for building the app on Windows (VS 2003) and Mac (xcode). It also includes a Makefile for building on Linux. For upper_case, this produces a separate .so containing the graphics part. The Makefile links some libraries statically (stdc++, glut etc.) so that the app will run on machines where these are old or missing. It also uses a technique where the main program exports its symbols to the graphics .so, which eliminates lots of the problems we had in Einstein@home.

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