Well I would first try to figure out what it is - it may be a chunk of cast. If its not cast it is probably mild steel. Tool holders for a lathe are one thing, you can also make with it more interesting stuff for your mill - if your mill is large enough to handle it - like some kind of angle plate. Or keep it for future - it will come in very handy some day.
I have Multifix B tool holders.
I did some grinding on it to start cleaning it up and I am pretty sure it is mild steel. I was thinking of using my mill to flatten it and square it up and maybe make a small surface plate out of it. Right now I am using on my hydraulic press as a base.