Here's one that's very dangerous. It has, on many occasions, resulted in the very sneaky inebriation of people who normally never drink too much. It's so smooth and yummy that it's very easy to drink too many.
The recipe was given to us by a group of Minnesota nurses who used to come stay and ski with us for a week every winter. They claimed the recipe was a group effort that had taken years of trial and error (the errant typically ended up under the table). It's also very easy to make:
1 6oz. can of frozen lemonade
1 6oz. can of frozen limeade
Use the empty lemon or limeade can to measure:
6oz. of tequila
3oz. of triple sec
1 egg WHITE (and it's absolutely critical you add this ingredient)
Dump it all in the blender, fill with ice, and pulverize.
We also learned through desperation late one night that you can substitute vodka for the tequila and end up with a very smooth, very drinkable alternative.

: