BWeld by Baidhir Hidair is a free script that creates welds at intersections of models. This is very useful for metal surfaces, where it's difficult to create the fillet welds that bring two pieces together. The script is really easy to use, and leaves the original selections intact. It can handle both curved and flat filleted welds. Flat fillets simply generate a loop of polygons that serve as a fillet or a decal on which textures can be applied. You can get the script here.