I found this beetle scampering along in my apartment the other evening, fortunately before my cats found it and everything turned into a scene of destruction and chaos. So I plonked it into my terrarium overnight so I could attempt to take pictures the next day. Alas, it was not very cooperative, and this was the best I could do (how do people get all those clear pictures of beetles apparently holding still?).
Joseph Fortier on BugGuide kindly identified it as a darkling beetle (Family Tenebrionidae), possibly something in the genus Eleodes (the desert stink beetles). As these guys eat fresh and decaying vegetation and I do not keep houseplants, it is a bit of a mystery to me what it was doing in my apartment, but they do seem to make their way inside sometimes.
As I released it outside, I spotted another beetle that looked to my eye the same, so I imagine it will do better out there.