A forge for blacksmithing or metal casting?
If you're doing a blacksmithing forge, you can smith all sorts of things. Nails, lattice work, shovels, spades, any kind of tool really. Blacksmiths were immensely successful people, and some still are.
As far as metal-casting, still plenty of things. A lot of it will require further refining, usually with blacksmithing or other metal work. I've seen people build their own lathes and saws with cast metal, tools, grills, anything really.
I'm not really sure what the difference is. Couldn't a forge be built for blacksmithing and metal-casting? I'd probably be smithing things such as various tools, cutlery, and the occasional piece of whatever here and there. As for metal-casting, I'd probably be able to find something to do with that as-well. I'm just looking for a hobby that has at least some real-world application, and metal-working is something that has always kind of appealed to me, mostly because it is awesome.