I'm planning to purchase some M60s to use as main speakers. I mostly listen to music, but with 5.1 music formats coming (DVD-A and SACD), I thought I ought to consider adding speakers for surround. I can get hold of some NHT SuperOnes for basically no money. Would it be OK to use these as rear speakers in a 5.1 setup, or do the front and rear speakers need to be identical to get proper sound? Thanks for the advice.

Although timbre matching can be vital to creating an equal sound all around, many people have mixed brand systems and have found a good integration for music or home theatre.

Take sushi for example, who has Hales System Two up front, the
Axiom VP150 as centre, Axiom QS8 for surrounds and a Hsu VTF-3 for a subwoofer. He has found that his Hales sound very similar to the Axioms so perhaps the sonic differences mesh really well for his preferences.

Personally i like to keep my front 3 speakers of the same brand and model types (left, right and centre) but certainly you can mix and match brands depending on how they sound.
Once again about the best thing you can do is try them with your system because only your ears will really tell you if it sounds good or not.

Free is good. Go for it. You can always upgrade later.

NHT SuperOnes are reputable speakers. I'd jump on them now. If you don't end up liking them as surrounds, you can always keep them for when you come up with an alternate use for them, e.g. remote speakers for music in a second room.