Fragmotion is a great tool for it, I really like it, easy to learn for beginners, and there are some tutorials available (I bought one from Arteria3D when it costed only 1-2 USD, and I found it really great, but you can find free ones too). it works with those free (and not free) .BVH mocap files, you can tweak animations even by scripts (very useful and simple!). it costs around 50 USD, handles a couple of file formats (X, MS3D), but its MDL7 importer/exporter is not fine (I reported it on its forum beside some other bugs). using it beside Ultimate Unwrap 3D it is very cool (what is for model skinning, and has a lot of file converting plugins).

(or you can use the cheap and easy Milkshape, but it has limited features in animating. it also has a lot of file converters, but never tested with BVH.)

and of course, you can try Blender. it's free and great, but very complex and its interface is not easy...