I work for RE/MAX so I’m biased, but remax.com has every property in the US that’s listed in the MLS. It doesn’t let you narrow down the search as much as I’d like and in my opinion it’s a little rudimentary, but you’ll find what you’re looking for. Every property, no matter which real estate company it’s listed with.
You can also try fsbo.com, forsalebyowner.com, and realtor.com. Ziprealty.com has a killer search engine but they’re only in a handful of states so you may or may not find anything depending on where you live.