I saw this program called PureRef, which helps me A LOT when I'm doing my digital art.

You can manage multiple reference images in one space without having to create new layers on your psd file, or having multiple windows open for various images. I find it super helpful for organising my workspace. It might not seem like much at first but once you get your head around it it is super helpful.

You can download it here. It's free, but you can choose to support with a pay-what-you-want system.

Also check here for a list of the controls/shortcuts which are super handy.