I am a great admirer of Starrett “Last Word” indicators. My collection is now quantity 4. These are lever type (as opposed to plunger type) indicators that look like they were designed in 1914. The range of measurement is only about 0.030 inch, but I find them to be very handy for alignment. The dial marks correspond to 0.001 inch, but are far enough apart that you can judge alignment to about 0.0002 inch.
Here is a 711F (top) and a 711C. The F is more versatile, having a lever on the side that reverses the direction of the tip and a threaded hole at the top of the body for attaching a stem (shown).
The indicators are very rugged and last forever, so there are normally a lot of them on Ebay. I have typically paid about $20 for a basic indicator. You also will see boxed sets with accessories (two typical accessories below) and these sell for more. My advice is to skip the accessories; I do not find them to be very useful. The accessories that I am constantly using are ones that I have made.
One difference among the indicators is the diameter of the ball at the tip. Here you see that the tip can easily be replaced by moving a spring clip.
Radial serrations connect the tip to an internal lever, and allow quick adjustment of tip angle.
Long Island Indicator has spare parts and offers a fixed charge to repair these indicators. They also give instructions on doing repairs yourself.
I make reel end plates by alternating mill and lathe operations, holding the material in a 4 jaw chuck. Both sides have to be machined, so I have to rechuck when I turn the part over. The indicator guides me to recenter the part.
Most often I center on an outside diameter, but here you see that it can also be done on an inner diameter.
The indicator is mounted on a homemade base made of aluminum, 1144 rod, and a swivel joint from McMaster. It is similar to the usual magnetic base for indicators, but the cross slide of a Sherline lathe is made of aluminum.
Here is a simple homemade accessory (two pieces of 1144 rod) that I use to center the mill spindle over a rotary table. The indicator dial faces away from me when I rotate around to the back side, so I keep a small mirror handy.