Since I am exercising, I was looking for a little input on sex and calorie usage.
Here's a humorous accounting of calories. sex calories humorous
MSNBC has the personalized calorie counter which bases its calorie usage on a 150 pound person. According to it, 30 minutes of moderate sex uses 46 calories (just sixteen more than sleeping) and vigorous sex uses 53 calories.
This page, Calories we burn whilst having sex!, seems to be somewhere between the two extremes above, but I wouldn't write it in my activity log.