Let us now praise sulfur. Well, some kinds of sulfur, anyway. The +2 oxidation state (earlier typo fixed, aargh) is a bit hard to handle, what with all those angry-skunk, burnt-tire overtones. But move up the ladder to +4, and you've got some possibilities.
Those do not include the hideous thioacetone, but bring some oxygen into the picture and you get a sulfoxide. And these guys I like, probably because one of the best compounds I've made in my career had one as a prominent feature.
Not every medicinal chemist shares my enthusiasm, that's for sure. Sulfoxides have a reputation for being potentially metabolically unstable - and they can go either way, being oxidized up to sulfones or reduced back to the parent sulfide. (I believe the former is more common, and is the clearance mechanism for DMSO, among other members of the tribe). But there are some out there in the market, chief among them esomeprazole (Nexium). Then there's armodafinil (Nuvigil), Cephalon's follow-up to Provigil, like Nexium another single-enantiomer-of-a-racemate drug.
But sulfoxides aren't just for extending your patent life and raking in the money. They can make a big difference in activity. The group has a strange character to it, because that oxygen atom is about as close to a naked O-minus as you're going to find. And the tetrahedral geometry of the sulfur means that this electronegative group is held is a very specific orientation relative to the other parts of your molecule. Like a nitrile, a sulfoxide is sui generis: there's nothing else that will do what it does.
And they're chiral. That can either be a bug or a feature, depending on your project and on your view of the world. If your target protein recognizes that chirality, it's probably really going to recognize it, because of that strong character. But that chirality is yet another reason that people avoid the sulfoxide, because that means chiral synthesis, which is a pain. All sorts of methods have shown up - chiral oxidation of sulfides, displacement with inversion at the sulfoxide sulfur - but there's no good general solution. The existence of the commercial drugs shows that this problem can be overcome, but there's no use denying that it's a problem.
All these problems can, at times, blend together. I was told some years ago about a Merck clinical compound that had a chiral sulfoxide. When they checked for metabolites, they found what looked like unchanged drug substance coming back out. A closer look, though showed that this was actually the enantiomer of the starting drug! What happened, as I heard it, was that the sulfoxide was first getting reduced, then oxidized back up to the opposite sulfoxide, when then passed out unchanged. Eating your starting material and collecting your own urine has yet to catch on as a sulfoxide inversion method, though. . .