Where is there a really large group of cars? I have a terrible urge to fire a missile or mini nuke into said large group and enjoy the ensuing fireworks!

Kinda northish..... well, in the general area north of Agatha's House. You will see a broken rail for an elevated train, that stretches east to west, or vice-versa, and underneath that you will see some vehicles.
You really need to scout the area before shooting, and kill anything in the area, avoiding the highway as the backdrop to your fired shots. Now walk along the road, both east and west. By scouting, you can find the beginning and end of the vehicle domino set up. I like to mine the first car, then shoot the stack with a scoped weapon. What is also cool though is mining a middle car, and watching the domino effect go in two directions. Really cool at night. Don't stand too close! I'd save a lot.

There's another one near the Sniper Shack, iirc, and again near the car fort... the drive in theater is pretty sweet. My most memorable explosion here (the one east of TPT) is dropping a gas canister in the middle of the lot while stealthboyed, then backing out and blowing it up.

For truly epic in a closed in situation... bring serious armor and gear, and go at level 25 or higher (Enclave needs to have been activated in BS by this time) to L'Enfant Plaza. An extremely target-rich environment, however, it's quite easy to die right away here. Like, right away. I'd save a lot.

Hey, how about the http://fallout.wikia.com/wiki/Abandoned_car_fort It's on a bridge, southish of Raven Rock

There is a string of cars in front of the Grisley Diner which is around the Temple of the Union, just a little south of Olney. Also, a very large group next to a raider fort between the Alexandria Library and the Nuka Cola factory.

Far south on the freeway that heads west from arlington library, at Flooded Metro there are a bunch of cars (and a bus or two) between the metro station and a raider encampment.

I think the best place for blowing up cars is the Overlook Drive In Theater

http://fallout.wikia.com/wiki/The_Overlook_Drive-In

Have fun!

Thanks for all the great suggestions folks!! I just finished the pit, I'm level 8, and I think I'll go blow something up!

I still can't forget my first experience with blowing up cars, I was shooting the raiders somewhere in capitol dc, and then suddenly all cars and buses around there exploded all, I was like ''wt hell another war is coming?'' .....and this is how I found out that the cars can explode when you shot them

OMG I totally forgot the Scrapyard! Went there this morning with my new character to get Dogmeat. There must be close to 80-100 cars!!!!!!!!!! Imagine that explosion hahahahaha

@ Elveora: Yea, I know what you mean! I did read that you should never seek cover behind a vehicle. So, while exploring the DC ruins and looking for the GNR building, some mutants fired at me. Where do I seek cover? The closes thing around, a parked car... OUCH LOLOL I think I had multiple crippled limps. LOL

I remember how I found out that buses can explode, with an even bigger bang than cars.

I was fighting some super-mutants and decided to take cover behind a bus, not such a good idea.