Anything, so long as it's mutually enjoyed & takes fewer than 3 days to physically recover from {ie no sprains, broken bones, transmitted std's}
Experience has taught me that I tend to most frequently want sex:

When I first wake up in the morning, or at least before I've involved myself in any of the day's projects
In the evening if I've been spending time during the day being attracted to & interacting with the person with whom I'd be having sex
Most of the rest of the time, if the interest is mutual

Experience has also taught me that I tend not to want sex:

Anytime I have the impression that the interest isn't immediately mutual
After eating a large meal (But I seldom object to eating a meal after sex.)
While I'm processing unpleasant emotional confrontation
If I feel like I'm not clean, especially if I've felt that way for a while

A few of the less mainstream sexual practices I've encountered on the internet sparked an interest. They include:

Tantra Yoga
M/F Spanking followed by sex
Some Bondage Photos (especially those artistic looking photos from Japan (and CaptiveCulture) & some involving hot wax) and videos of women in bondage experiencing orgasm

The bulk of my interest is (& all of my experience) has been, however, fairly prosaic (or so it seems to me).

Note:

I consider sex to be a social activity, so my aforementioned uncomfortableness with interacting in large groups of strangers applies here.
Note also, as of June 2014, the last two times I had sex with another person were in April 2006 and April 2000(why?). I doubt I'd be much good for (non-novlety, I guess) casual sex without a fair amount of "practice sex" or "rehab sex" first.
Another note: If you're a woman I'd find attractive and you meet me in person, most likely the following feelings will be at conflict within me:

The instinctive desire to remove your clothing and penetrate you with my penis.
The experience based desire to avoid interacting with you because doing so will consume lots of personal resources and ultimately leave me with unhappy memories. This desire best achieves its goal with behavior that gets you to avoid me.
The imposed from above decision to be polite, informative if it's information I think you'll find useful, and a little distant. This will be in control, but it won't quite mask the presence of the other two.