The NASDAQ heatmaps can provide a quick snapshot of what's hot and what's not.
It's a free tool , and you can create custom heatmaps by loading your own selection of stocks or ETFs. The premarket heatmap provides a good indication of what's likely to come, at least for the first 30 minutes. Also useful for providing a quick sector analysis.
Not real time, but updated every minute from 15 minutes pre-open to 15 minutes post close.
The ETF heatmap is shown above, but other canned sorts of the NASDAQ components are available on the site.