As a blast from the past, Poseidon bundles one of the last good versions of Firefox ever to be released. Yes, I just said something bad about Firefox, Mozilla totally deserves it.
Not much really to say here, it works identically to Ubuntu.
Poseidon includes quite a large number of screensavers by default, including the ubiquitous Flying Toasters.
Rather than the regular Synaptic Package Manager, Poseidon includes this nice add/remove utility. Manual package installation is identical to Ubuntu.
Interestingly, Poseidon bundles Virtualbox by default.