Briefing.com. Doesn't really matter to me tho since I only watch two sectors; semi-conductors and biotech. -FastTrader
I use my data provider and my charting sw. It is easy and I see every minute what sector is strong and what is weak for a day.
I use Quotetracker and IQFeed as provider. You need to know symbols for each sector and then you can receive data from your data provider and sort them any time (the best way is to wait minimally 5 min after market open to settle the prices little bit) in order you want. Here is sample screenshot.