SmillaEnlarger is a single-purpose app that enlarges images without leaving them blocky and indistinct. Its algorithm obviously isn't going to do a perfect job of filling in the details, but it draws smart curves that actually look good at large sizes. Just as importantly, it eliminates the artifacts and pixelation that usually show up in enlarged images.
The display features the original image and the enlarged result side-by-side, so you can easily see which part of the source image you've selected, and you can crop and reposition as necessary. You can adjust the size of your output image either by using a slider or by entering the target dimensions. SmillaEnlarger is not a full-featured image editor, but there are a few options for adjusting sharpness and noise. It's a free app, but it would be worth paying for to anyone who has to manage a lot of frustratingly tiny photos.