Can Angels Appear in Human Form?

Most of us have heard the idea that angels can take human form. I remember hearing this from time to time when I was younger, related to the verse from Hebrews (ch. 13:1-2) that says, "Let the charity of the brotherhood abide in you. And hospitality do not forget; for by this some, being not aware of it, have entertained angels."  Old Testament Manifestations
 This verse is a reference to the moment in Genesis when three visitors appeared to Abraham, in Genesis 18 verse 2.  The Lord appeared to ...