two words mate; "sprue" & "rubble"!
i have a big box of emptied sprues that i'll cut up into rough bitz and then use them for basing my guard army to make it look like they're fighting in a urban hell!
also very handy if you're planning on making terrain! a few piles of sprue rubble can really help to finish off a ruined city.
for those very few and rare 'rounded' sprues, cliping up those can easily be used to convert snipers or standard bearers and such!
the main thing to be carefull of is avoiding the areas of sprue that have the part codes on them!
cheers!