Where to advertise and how to advertise probably depends most upon what you've made, where you want to sell it and who the intended "targets" are. Unless it is really interesting for "the average 805x programmer/user" there's little chance that you'll sell anything on a special interrest forum by putting on (paid) advertisement banners. Anyhow, there may be lots of forums on this particular subject where you can find loads of information on this subject.