Windows only: Radical Image Optimization Tool makes it easy to see how compression affects the image quality of the pictures you're optimising with a handy side-by-side interface.
Sample photo by XirannisX back On.
Radical Image Optimization Tool isn't the best choice if you're looking to bulk re-sample images, but when it comes to squeezing quality out of every byte, it does let you exercise a high degree of control over the compression of an individual image.
Your images are laid out in a side-by-side format with immediate updates for any alterations you make to the level of compression or other settings. The file size of both the source image and the optimised image are also displayed above the images, so you can see immediately if the quality-to-size ratio is acceptable for your purposes. If your need isn't so much tight control but bulk converting, check out how to batch resize with Infranview and JPEGCrops, a speedy batch cropper that lets you specify crops on a per picture basis. Radical Image Optimization Tool is freeware, Windows only.