You can also buy Vanguard funds in your Fidelity account (for a fee).

One of them that I have (IVV I think?) has a 0.07% expense ratio, so I think that's as good as any Vanguard fund.

Yeah, Fidelity has a deal with iShares where you don't have to pay any transaction feeds with their ETFs. There is a whole section if you poke around on the menu. There are a couple different S&P 500 ETF options.

Is there an S&P index fund or ETF on Fidelity? I know Vanguard has a few of these, but given that I already have a Fidelity account, I'd rather not open one at Vanguard as well. Names would be appreciated as I can't seem to find them by searching, though I know they have to exist. Nothing fancy -- just a fund that tracks the S&P and therefore (hopefully) has low expenses.