I have one of those - it is called a ganglion cyst. I had my ortho check it out. He said he could do a procedure to get rid of it, but it could come back. From what he said - perfectly harmless. It looks a bit disgusting and I may have him deal with it, but no need to worry.
My doctor is very highly regarded too. He was choosen to represent the east coast as the ortho. working the xgames in Colorado a couple of months ago.