I find the Zero Hedge website a good one to watch, and at least be aware of. "Tyler Durden" has a, well, cautious if not skeptical take on things that is worthy of monitoring.
After today's stock swoon, the website is taking note of how financial stocks are leading the way in recent weeks. That post also raised questions about how recent developments may or may not be similar to those before the 2007-2009 financial crisis -- including rising counterparty risk among large financial institutions.
With financial stocks weak, it might be worth staying on top of conditions in money markets, for a clue to how stressful things really are.
... including overnight borrowings among the banks. The "LIBOR" (London Interbank Offered Rate) has been rising recently, albeit nothing like the spike that was reported in 2009.
The LIBOR rate, of course, came under some question for its construction, and the honesty of reporting. Wonder if they've really fixed that.