Is the "wit" in "to wit" the root of any other English words?

"witness" is one.

As you already mentioned "to wit" is from an old Saxon root. I can see some link with the German "wissen" (also to know), Dutch "weten" and (I'm told) Danish "Vide" .

As in many languages "to see" and "to know" are interrelated concepts. So that "to wit" is not only about knowledge but witnessing.

On the knowledge side, it is also easy to see the link with wise and witty and wittingly.

The "Witan" being the Anglo Saxon assembly of wise men - knowing enough to have their word in the destiny of the community.

"wittingly": knowingly.

update
Following mplungjan's comment, I read that the common root would be the Sanskrit "veda" (knowledge), which also yielded Latin "videre" (again "see" and "know").
All of these forms (including the Sanskrit) come from the Proto-Indo-European word *weid- (credits to Kosmonaut)

Witless http://dictionary.reference.com/browse/witless

In addition to ones already mentioned:

inwit - mind, reason, intellect, understanding
witcraft - logic, reasoning
witful — wise; sensible
witling- a person with little wit
witter - knowing, certain, sure, wis; to make sure, inform, or declare.
witticism - a witty remark
wittiness
wittol - an acquiescent cuckold.

There are more in Middle English that I don't think are to be found in Modern English.

The word wit is from Old English. According to Etymonline:

"know," O.E. witan "to know," from P.Gmc. *witanan "to have seen," hence "to know" (cf. O.S. witan, O.N. vita, O.Fris. wita, M.Du., Du. weten, O.H.G. wizzan, Ger. wissen, Goth. witan "to know"); see wit (n.). The phrase to wit, almost the only surviving use of the verb, is first recorded 1570s, from earlier that is to wit (mid-14c.), probably a loan-translation of Anglo-Fr. cestasavoir, used to render L. videlicet (see viz.).