They are not a preffered choice these days by many. Things like Planks, Roman Chairs, Leg Lifts, Reverse Crunches etc and secondary activity from things like Deadlifts, Squats, Cleans etc, are seen as more desirable forms of Abs stimulation these days compared to sit-ups.
There is a form of sit-ups called Janda Sit-ups, which were developed by a Doctor Janda and Pavel Tsatsouline a few years back, that require two people. Supposedly the technique gives the lumbar region a more preferable type of interaction, when you work the Abs, to reduce the risk of Lumbar side effects.
There was also an associated product called the Pavelator, designed mostly by Tsatsouline as, I believe, a way of doing Janda Sit-ups on your own, but It may have just been a gimmicky product, though I'm not saying that for certain.
There is another type of sit-up called the punch sit-up, where you tag a couple of cross jabs on the end, before lowering the midsection down again, but I think some may consider those on a low par with regular sit-ups too.