The easiest to make yourself is the shovel handle seater. The seater has a handle that pivots on a block with a notch cut in it to catch the back of the mortise. Slightly above the block is a rod that will pivot. You can use a plugged case or build a fancy one where the plug slides and lets you put the bullet in the case. Either way put in the bullet. Insert the plugged case. Put the rod on the handle at the end of the case and use the handle to seat the bullet. The pros it is easy and cheap to make yourself. The cons it is two pieces.

40 Rod

  The two easiest I've found to use, and that you can make yourself, are the plugged case, one of which came with the Schoyen-Ballard I have. The other one is like the hand seater Lyman used to advertise. Both of course require a tapered bullet so that not much effort is required.

  For straight sided bullets you'll need a mechanical seater as 40_Rod and Crash mention. Unless you have a machine shop available you'll most likely have to buy these.

  I've also made up one that uses a modified plugged case that can be adjusted for seating depth. It was made up so the bullet can be seated down inside. A simple handle, with a swiveled rod attached, that hooks into the rear tang sight base is used to seat the bullet. Kind of hard to explain but can provide a coupla pictures if you'll send me a PM.  When using the tang sight I'll switch over to the Lyman copy.

PETE

The one I use with my Schoyen Ballard looks like a file handle with a offset round rod about 1/4" diameter, and a plugged case on the end. It seats my Ideal stepped bullets without much work, but after 40 or so shots it can get a bit tougher on the wrist.