[Solanum]

A Telrad. It makes aiming the scope so much easier. Once I got one I really wished I'd got it with the scope because it would have saved me a lot of squatting down on the ground peering along the tube of the scope and then doing it again, and again...

The nice thing about an actual Telrad, as opposed to the other red dot finders is that it is easy for the beginner to mount it (just sticks on the tube). Most of the others need an extra plate putting on or removing the finder. I find the magnified view in the finder is a big help for finding stuff that isn't visible to the naked eye, so I don't want to remove it.

EDIT: you will of course need some means of collimating first... but I'm afraid that goes without saying.