This seems to have been a very popular first, the Ruger Single-Six:
This one is the New Model Super Single-Six, to use the whole thing. Big difference between the new and old models was the old was like the Colt SAA; if you wanted to carry it loaded, you had to leave an empty chamber under the hammer. The new model uses a transfer-bar mechanism to allow carry with all six loaded; if it hasn't been cocked and you're not holding the trigger back, the transfer bar is down and the hammer cannot strike the firing pin. Offhand, I don't know if the old model had adjustable sights, new model does, nice square ramp in front.
It also, as Mr. Completely noted, comes with a .22lr cylinder, and another chambered for .22 Magnum. Push the release, pull the cylinder pin, trade cylinders, replace pin and go back to shooting. Now, the bullet for .22 Mag is slightly larger diameter than .22lr, so the bore diameter is a compromise between them. While that probably has some effect on accuracy, it's not enough to worry about unless you're a serious bullseye shooter, and even then if you find the right ammo... This group was shot with Eley Sport from ten yards:
For me, after several hours with centerfire rifles, that's pretty damn good. And no, I didn't remember the magnum cylinder, so I don't have a comparison to show. Next time.
I have no idea how many rounds have gone down the barrel of this, first from me and then from my kids. It's solidly made, reliable and accurate, and for teaching kids it slows them down a bit since they have to cock it for each shot. And it's got one more neat thing: since it was made in 1976, stamped into the top of the barrel is 'Made in the 200th year of American Liberty'. Ruger didn't advertise it, just did it.
I'll agree with the Mr.: if you can pick one up, you should. You'll like it.