bliss, the aim of which is exactly this - a single web based tool for organising a music library. While it appears to be in the relatively early stages of development, with an initial early focus on Album Art management, it works very, very well and I'm keen to see how it develops.
I won't cover the functionality of bliss here, there are full detailed instructions on the bliss website, but here are some instructions specifically for installing bliss on a ReadyNAS device.
Note: as bliss is a java based application it can only be supported on x86 based NAS such as the Pro and Ultra series. Sparc based devices like the Duo & NV+ are not - and will never be - supported
(Update: 11 June, 2011) I have now written a ReadyNAS addon to simplify installation - details can be found at http://sphardy.com/web/bliss