A free, live S&P 500 heatmap. Every constituent is a tile, sized by index weight and colored by performance, so you can read the whole index at a glance. No account required.
The S&P 500 is also tracked by the three largest US ETFs: SPY (SPDR S&P 500, about 570B in assets), VOO (Vanguard S&P 500, about 515B), and IVV (iShares Core S&P 500, about 480B).
See the matching ETF heatmap or the ETF screener.