22s are a metric buttload of fun, and I'm planning on adding several more to my collection, as I only have one right now. My next 22 will likely be a bolt action CZ-452.
I'll second the Ruger Single Six. If you want to shoot 22 Magnums, with the ability to shoot ultra cheap 22LR in the same gun, you really can't beat a nice used Single Six. I've got one on my wish list. Either that, or if I come upon a good used Heritage Rough Rider Combo for $80 - $90, I might just buy one of those. Had a Heritage for a while and loved it. I stupidly sold it, so one day I'll replace it with another Heritage or a Single Six. The Heritage guns are typically just as accurate as the Rugers, though they are crudely finished and quality can be hit or miss.
Another pair of really nice semiauto 22 LRs are the Browning Buckmark and Smith & Wesson 22A. I'm not familiar with either of these models, but both are spoken of very highly here on THR.
My favorite 22 is the one I currently own, a S&W K-22 / Model 17 Target Masterpiece. A true classic. And the DA trigger gives me lots of good, solid practice in the art of trigger control. A lot of people don't see the need for a DA 22, but to me the make excellent training devices. If you can shoot a DA revolver well, chances are that skill will translate to your other guns. And they are a ton of fun too. Prices are getting high nowadays, and they are becoming more difficult to find. By no means impossible though. Expect to pay $350 for a "shooter grade" K-22, and upward of $500 for a near mint condition example. But they are worth the price. Colt Diamondbacks in 22LR are a direct competitor to the K-22s, and fine revolvers in their own right. But they are even pricier, and can be tough to find.
As for semiauto 22 Magnums, a few have been made, but I don't recall the make/model.
And BTW, the only guns that can really shoot both 22 Mag and 22 LR are the single action-type revolvers with two separate cylinders. I've heard of people shooting LRs from DA 22 Mag revolvers, but I wouldn't try it, personally.
My S&W K-22, M17-0. 1959 Mfg date.