One of the things that pisses me off is when someone uses open source and don’t give the credit to the authors.
This happens a lot with web applications.
Several times I looked at some site and thought “hey, that’s a nice engine those guys got there”, only to find, in some weird way, that it is just a theme for WordPress. Honestly, that’s probably the least credited open source project ever (well, maybe after Apache and PHP, anyway). Several themes around remove the “powered by WordPress” completely.
Come on! It is not that hard to add a simple “powered by” in the bottom of the page…