Originally posted by Mr. SlackPants
Most off the time that happens when your grid size is larger than something like 32. If you try to add stuff in the 3d view it doesn't work so lower the grid size or do as ertai said, use another viewport. But remember that the object you'll place stays on one of the 0 lines. So if you add it for instance in the top view the opject will have the x,y coordinates where you place it but the height will be 0.
Sometimes setting the grid size lower doesn't work.
Oh yeah, it's ertai23, not ertai