Joe Van Dyk suggested that:
There should be a lot of code out there on Ruby extensions written in C.
Pierre, check out the standard library, which comes with Ruby.
YAML/Syck fits the glue/extension category - Syck is a C extension, YAML is
the part written in Ruby.
You can also quickly and easily write wrappers for binary libraries in pure
Ruby, using the DL library. There are no examples of that in the standard
libary, I think, but they exist on RAA, and I can send you one I made for
AutoIt for Windows if you're interested. (Wrapping AutoIt this way is
redundant, because it provides a COM interface.)
You might also want to check out the Ruby Quiz, which is mostly not that
scientific, but includes different people writing code for the same goal,
and a summary of the solutions given; see what titles strike you at