Working out if a particular file being offered for download is 'legal' is a tricky process. Quite often you may want to assume a song has been made available with the copyright holders permission, but it's very hard to tell for sure. However, as a rule of thumb, if you can find a piece of music for sale somewhere (by a legitimate looking organisation) it's probably being offered for download by some well-meaning but ultimately lawbreaking fan of the band or singer in question.

In most nations, you'd be comitting a similar offence to downloading and listening to such a track if you recorded a song you liked off the radio, or lent a CD bought from a store to a friend, or even played such a CD to a gathering of more than a handful of people. In the US in particular music is more about money than art, and those standing to gain most by restricting fans access to music are trying very hard to stop people sharing music freely. This is the cause of the lawsuits you might have heard about in the press - although these have generally been targetted at the fans who are offering the music for download rather than those downloading it if I understand matters.

In any case, LimeWire itself is legal - it's just a bit of software, and like any bit of software it can be used for both legal and illegal things. The choice of which is up to it's user.