Sigma APO 1.4X Teleconverter

This is a great teleconverter that's not very expensive. Yes, you lose one F stop of light, but with a Digital SLR, it makes no difference. Most high end DSLR already have a built in mutipiler effect, so add the that to a 1.4X and your telephoto lens are hitting the high end in range for much less the cost of buying a 600mm F/4.