DKJ-Crop
DKJ-Crop is a free hotoshop script/action for cropping photos.
Isn't that already built into Photoshop, you may ask? Yes, but DKJ-Crop comes with a lot of convenient sizes that you otherwise would have to define as presets. In particular, DKJ-Crop "knows" about the print sizes from Prints by DKJ so you don't have to remember dpi settings, colour profiles and how much to add for trimming and canvas wrap-around. Also, DKJ-Crop informs you whether the photo needs to be enlarged.
Download and installing
To download DKJ-Crop, simply click on the download button. This will download a file install-DKJ-crop.jsx.
To install, double-click on install-DKJ-crop.jsx. This should launch Photoshop and ask if you want to install an action for DKJ-Crop in the actions panel. (If you don't see the Action palette, you will find it under Window > Actions.)
If double-clicking doesn't work, you can also install DKJ-Crop by doing File > Scripts > Browse... and choosing install-DKJ-crop.jsx.
Using DKJ-Crop
Open a document and run the "DKJ-Crop" action. It features different tabs for different uses, the first three being prints offered by DKJ. For Fine art prints, an extra 3mm is accounted for so that, e.g., a 15x20 cm print will be cropped as 15.3x20.3 cm.
For canvases, there are two types of frames, 16mm and 40mm. DKJ-Crop accounts for the wrap-around and crops, e.g., a 40x40 cm canvas print as 49x49 cm (if the thicker frame is used).
After the crop, guidelines are laid out to show where the photo will be trimmed/wrapped.

Video: Installing and using DKJ-Crop.