The simplest useful program that uses Erlang/OTP features I could think of. Website monitoring needs concurrency (Erlang) and reliability (OPT) . This OTP server monitors a list of sites and shows a heartbeat when all is well and a warning when one goes down. It's mean to be basic but email, twitter and lava lamp support may follow :)
It works with the rebar pre-built binary version and erlang r14. Use Kerl to install particular versions of Erlang/OTP. It is like Erlang's answer to virtualenv from the Python world.
To try this out just clone this repo, cd to the /rel directory and do a
rebar -v clean compile generate
Rebar packages everything into a single binary
rel/erlyup/bin/erlyup
run it by doing a
rel/erlyup/bin/erlyup console
The only files that are of interest (non-boilerplate) are the server that does the work
src/erlyup_server.erl
and the OTP supervisor that manages it
src/erlyup_sup.erl
Rebar does have some issues working with particular versions of Erlang but I can do a version of this for the major versions of Erlang if someone is interested. Some useful coverage of rebar.