The Supreme Court can rule that an Executive Order made by a President is unconstitutional. And Congress can effectively nullify an executive order by not funding the action ordered.
I think Trump is up to 11 executive orders now. The first one, as you may know, didn't or didn't do something to Obamacare, it's not yet clear. The most recent one I know of orders that that wall be built (I'm not sure whether Donald has actually ordered Mexico to pay for it), and also orders tougher measures against illegal immigration. Others have rescinded funding for abortions provided by international organizations, put gag orders on EPA employees and peer-reviewed articles by USDA scientists, and approved construction of the Keystone XL and Dakota Pipelines.
Do we still have checks and balances in this country? How extreme and crazy will Trump get before (a Republican-majority, I know) Congress and the (deadlocked, 4 to 4, I know) Supreme Court begin to oppose him?
This page on Paste Magazine's website says they'll be keeping track of all of Trump's executive orders. I'm not familiar with Paste Magazine or how well they keep their promises, but that's what they say they'll do. They're at least one order behind as of 3 PM, 25. January 2017: they don't mention the order about the wall yet. Maybe the page is a one-time thing, not a running list updated as we go.