The only thing I've found that's close is this music player called Enqueue (http://www.enqueueapp.com/features.html) It has folder monitoring built in and I used it when it was in beta, but now it has to be bought, plus there hasn't been any updates to the program in 2 years. I wish this was easier.
A couple neat things are FLAC/OGG support, folder Monitoring and a play queue. The screenshots on the website are actually a little old.
iTunes on OSX is not bad. The upcoming iTunes 12 update is only more improvements too. The only thing it's missing is FLAC support really. If you use an iOS device it's pretty much your only option too. I don't like how it can only read album art from embedded ID3 tags too. Seems kind of dumb.
If you did want to swap, I'd highly recommend Enqueue. It supports FLAC natively (and other odd, open source formats), and native Scrobbling to Last.FM. You may find it more lightweight than iTunes too because it doesn't carry all the store front end bloat, which is essentially a really shitty web browser. No QuickTime shit having to load either (think videos/trailers, it loads even though you may only be listening to music as far as I know). The only thing it won't do is sync to a modern iOS device. There's nothing that can really other than iTunes. Apple keeps that shit locked down.
So it's up to you really.
The only thing I've found that's close is this music player called Enqueue (http://www.enqueueapp.com/features.html)
It has folder monitoring built in and I used it when it was in beta, but now it has to be bought, plus there hasn't been any updates to the program in 2 years.
I wish this was easier.
I use foobar2000, which is great but Windows only. I quite like the look of enqueue as a mac OS alternative, or maybe Songbird. Not sure how well these would work with an iPod though.
Ecoute is very good (I bought it), but I would not call it a true iTunes alternative because it still relies completely on the iTunes database and even opens iTunes automatically for you when things need to change.
Enqueue would be my recommendation because it is a true stand-alone replacement with its own library.
iTunes is only resource-heavy because it does everything. Enqueue just plays music, not try to sell you anything.
http://www.enqueueapp.com/ - Made by a Redditor
http://cogx.org/ - Pretty bare bones
http://doubletwist.com/ - Good if you have Android, I believe this was made by DVD Jon, the guy who cracked the DVD and iTunes encryption.
Wow, I can't figure out how to submit links for the life of me. Anyways it can be found here. screenshot 1, 2