What's the easiest way to add in printers crop marks in InDesign or Illustrator?
For when the job goes to print - they need to see where they need to cut.
I agree with @Alex -- if you have just a single page, artboard, or document you need output with marks, just save/export as a PDF (although I'd suggest PDF/X-1a with marks).
If you are working on several items on a single artboard in Illustrator, you may want trim marks around each item or set.
If that's the case, grouping each set then choosing
Effect > Crop Marks will add crop marks to that set of objects.
Like all Illustrator effects, the Crop Marks effect can be applied to objects, groups, and layers. So you can choose what exactly gets read for the marks.
The effect reads the edges of the objects/group. So, it places crop marks exactly at element edges. If you want a bit of space between the objects and the marks (margin) then you need to create a rectangle the size of your desired trim area and then apply the Crop Marks effect.
This will allow you to move the objects or group around and the crop marks stay with it, like any Illustrator effect.
(I can't test this, but I believe in pre-CS6 versions of Illustrator the Crop Marks was a filter, or perhaps an Object Menu item.)
In InDesign, export as 'Adobe PDF (Print)' and then in the Export Adobe PDF dialog box, under 'Marks and Bleeds' tick the 'Crop Marks' box.