Which S&P 500 Index Fund or ETF to Use?

Q. I want to invest in S&P 500 index, which fund should I use?

A. For investors who want to build a diversified passive investment portfolio, a S&P 500 index fund is an essential component (it is better than Nasdaq index fund).  The problem is, there are at least a few dozens options available to invest in S&P 500 index, most are mutual funds and not worth the expenses, some are ETFs.  Here is a list of the S&P 500 index funds worth considering:

Good S&P 500 Index Funds
Morningstar Gold Rating - Fidelity Spartan 500 Index (FUSVX), Vanguard 500 Index (VFIAX), both charge 0.05% for investors who meet the $10,000 investment minimum. 

Morningstar Silver Rating - Schwab S&P 500 Index (SWPPX), expense ratio 0.09%, but you can start with only $100 investment.  

S&P 500 ETFs
ETFs require no investment minimum, but there is trading cost. 
iShares Core S&P 500 (IVV), expense ratio 0.07%
SPDR S&P 500 (SPY), expense ratio 0.09%
Vanguard S&P 500 ETF (VOO), expense ratio 0.05%, but this ETF is technically a separate share class of the Vanguard 500 Index mutual fund.