Slate, sandstone-I have some awesome fossilized sandstone burrows that are tubular and great hiding places for fish and shrimp, granite, gneiss, that cool holey black Oregon basalt, limestone especially the holey stuff, pretty much anything that looks cool and isn't crumbly or full of something that is possibly harmful. I know there are some cool river rocks in Northern California. Around here we also have big fossil oysters and ammonites that look cool in a tank. If you don't know what something is, do some research on local rocks, and if still stuck, go to a local university and someone will help you out.
Some people worry about limestone altering their pH... It might buffer your water a little bit, but it breaks down relatively slowly compared to some other stuff. I wouldn't pack a soft water tank full of it, though, because you will be fighting raised water hardness.