e.g. to "copy" another objects x coordinate onto the current object without also moving the current object in Y or Z:

1. put the cursor on the desired X coord
2. select the object you want to move
3. change rotation/scaling pivot to "3d cursor" (period key)
4. turn on "move object centres only"
5. scale in x (s key, then x key to restrict to x)
6. type 0 and hit enter