Clementine:
Free
Clementine wins first place in my search for a lightweight iTunes alternative. It doesn't have as many bells and whistles as Songbird but those bells and whistles are what makes Songbird a little bloated. I love that it has last.fm support built in at fresh install.
- last.fm support
- lightweight
- multi-platform
- sync any device
- library scanning
- folder based browsing

Songbird:
Free
Second place goes to Songbird. It's a little bloated but it has a much smaller memory footprint compared to iTunes.
- ipod support (and any other media player)
- can do anything that itunes can do plus more
- themes (feathers)
- tabbed browsing (I don't use this but might be useful to some?)
- incredible blog/podcast support
- codecs... we all know Itunes blocks everything and this is the main reason why I hate it.

Ecoute
Commercial ($10 license)
Ecoute has the most beautiful ui I have experienced in this epic search. It reminds me of the foobar theme I use (monolite... maybe that's why I like it so much).
- integrates with last.fm, facebook and twitter
- themeable
- supports apple remote



Vox:
Free
For the folder based user that doesn't need any social addons or device syncing. It's extremely lightweight and will handle any audio format you throw at it.
- lightweight
- nice looking ui
- no dynamic library functionality
- no last.fm support
- includes numerous effects like equalizer, reverb, time stretch, pitch shift, echo
- all supported files can be exported to aac+, apple lossless, wav and other formats with enabled effects.



Instinctiv:
Free
The most social networking savvy of the bunch... interacts with facebook, twitter and last.fm. It has a beautiful user interface... It has “cover art cleaup” which actually kind brings down the app many points in my book; only because the user can't choose where to pull the album art from (i.e. cover.jpg, folder.jpg etc). It appears to only take the album art from the id tags. I should probably note that not everbody will care about this. In fact if you want an app to grab album art for your library on its own, this is the app for you. It also gains points because it's multi-platform.

Decibel
Commercial ($33 license)
This one is more of a basic audiophile's player...
- can take exclusive control of the output device (hog mode) and send audio in the device's native format.
- can automatically adjust the output device's sample rate to that of the playing track, preventing audio quality degradation associated with software sample rate conversion.
- minimal, non-obtrusive interface
- one click library import

Everplay:
Commercial ($14.95)
Everplay is basically a lightweight version of iTunes. Because it uses your itunes library it will play the same audio formats as itunes. If all you are looking for is a smaller memory footprint and don't care about locked down codec options... this is the way to go.

Cogx:
Free
Last but not least... another one for the folder-based listener.
- simple ui
- last.fm support
- lacks album art support
- very lightweight
- development seems to have stalled but it is still stable
