Off the top of my head - five dances on film that occur naturally in the plot:
1 - Uma Thurman and John Travolta in Pulp Fiction ('It was a teenage wedding and the old folks wished them well')
2 - The Full Monty in the Dole Office when they dance to 'Hot Stuff' by Donna Summer (it only lasts a few seconds but I think it's great)
3 - Harrison Ford dancing with Kelly McGillis in the Amish barn in 'Witness' ('Don't know much about history, don't know much biology')
4 - Michael Madsen in Reservoir Dogs doing that little dance to 'Stuck in the Middle with you' before he cuts off the cop's ear
5 - Condemned man watches Fred Astair 'dancing cheek to cheek' in The Green Mile
Can you think of more so I can make a top ten?
What about dancing that isn't really integrated into the plot (for instance the wonderful 'Commence to dancing' by Laurel and Hardy, or 'Singing in the Rain' ?)
Ha - looking for the above I found a Pulp Fiction/ Laurel and Hardy mashup here.
ETA Putting on the Ritz here Check out when he slows it right down about 1:30 into the clip. But the whole thing is fantastic.