After trying many applications and plugins, finally I found the best way to read lyrics of my songs when they're playing. The osd-lyrics project is an standalone application that's compatible with most of music players in linux, eg: Rhythmbox, Banshee and Amarok 2.

osd-lyrics shows song's lyric directly on your desktop, so you can read it everytime you need. osd-lyrics is completely configurable, you can set the font family and size, color and many more options.

osd-lyrics is working in two ways: OSD and Scrolling, in OSD mode you see only the sentence that's playing on your desktop, but in Scrolling mode you will see previous and next sentences too and the text will scroll up when the song is playing.

osd-lyrics is working synchronosly with your player, so when you pause the music it will pause, and when you seek the music to middle it will seek the lyric too.

Enjoy it: http://code.google.com/p/osd-lyrics/