Dick Hallorann appears in The Shining and in It, and of course It has multiple connections to DT.

Mother Abigail in The Stand makes a subtle reference to the power known as shining ("My own grandmother used to call it the shining lamp of God, sometimes just the shine." )

"The Raft" definitely contains a connection to Dark Tower. It appears that, much like The Mist, something from Roland's world has come through to ours. An image of the man-eating blob was what came to my mind the instant I read about an army of men devoured in the thinny outside of Mejis.

Now, I am not sure if this is a stretch, and I only include it because R.F. is also Walter in The Dark Tower series. In The Stand, R.F. gives black rocks, which in some cased are also things the person really needs. You may already see my connection here. After giving them these needed things, he has this power over them... Needful Things.... Sounds very familiar right? I know it is not exactly the same, but very very similar.

The villain in Storm of the Century is named Andre Linoge.  It is pointed out in the book that this is an anagram of Legion, which is the true name of the Ageless Stranger according to Walter at the end of the Gunslinger.

Also, on a smaller scale, the townsfolk of Little Tall island tend to use radio channel 19 to contact each other.