I discovered this about a year ago when searching for something to convert between PAL and NTSC video, and this totally saved me. There are other methods, but they're WAY too cumbersome (check out Apple's directions for using iMovie to do it). This is great. You just select "Standards conversion" and it chugs away, producing a perfect conversion on the other end. If you create chapters in iMovie, it even preserves those in the new conversion. Drop it into iDVD, and you can now burn DVDs in both standards.

AND... it's FREE!!!