1. Armbar from the closed guard by Ricardo "Cachorrão" Almeida:

“Now let’s analyze some shortcomings you might encounter and some tips that will help. The armbar from the guard makes you vulnerable to a pass of the guard; make sure you always adjust your hips after any missed armbar attempt. You are exposed to the slam; develop the habit of hooking the inside of your opponent’s leg with your arm so you don’t get picked up and slammed. Especially in the armbar from the guard, because your opponent is on top, gravity is working against you. Whenever someone tries to stack me, I like to turn belly down to use gravity against my opponent’s arm.”
Here's 2 vid's showing slightly different variations of this technique:
2. Mount by Saulo Ribeiro:

“I train this fundamental daily and, as a hint, I suggest you try to keep the mount without using your hands, for that way you develop both your hip game and your balance, not necessarily having to hold your opponent with your hands. Your biggest challenge is searching for efficiency and not diminishing the number of people who can escape your mount, so don’t worry so much about holding them down when training.”
3. Tight guard pass by Fábio Gurgel:

Here's a short clip on a close pass that looks a lot like an x-pass variation:
4. Taking the back by Marcelo "Marcelinho" Garcia:

Let's just toss up Marcelo vs. Shaolin from the 2003 ADCC's for an example:
5. Loose guard pass by Vítor "Shaolin" Ribeiro:

Here's a short clip on a Spider Guard pass:
6. Kimura by Rodrigo "Comprido" Medeiros:

*Trivia: The Kimura arm lock used in BJJ is one of only two techniques named in honor of a Judo player. This bent arm lock was a favorite of (Kosen) Judo and Jujitsu master Masahiko Kimura, who fought Helio Gracie in 1951.
The Kimura is a versatile submission, and in this clip, Stephan Kesting from Grapple Arts shows how to get it from the Half Guard:
7. Triangle choke by Marcio "Pe de Pano" Cruz:

In this clip Dean "The Boogeyman" Lister shows some very nice details about the triangle:
Here's a nice little variation of the triangle set up:
8. Sweep from the guard with the opponent on their knees by Xande Ribeiro:

The other detail is the positioning of the base-foot; it must be fixed on the ground and close to the opponent’s shin. By raising the foot, the lever is lost. There is a variation, putting that very same foot on the foe’s hip. The secret to this sweep is the timing, for if you just execute it, the adversary will be rigid and strong; so it’s necessary to feel they are relaxed and surprise them right then. Another advantage of this sweep is that the opponent doesn’t feel comfortable moving, which makes them, in their anxiety, leave room for other attacks.”
I edited the Ribeiro Brothers HL vid from ZenMonkey to show just the sweep that Xande is talking about in this clip:
9. Rear-naked choke by André Galvão:

Stephan Kesting from Grapple Arts has put together this very nice tutorial on getting the Rear Naked Choke. He also has some great details on the harness hold as described by Marcelo Garcia (above):
10. Pulling guard by Roberto Roleta:

I'm not a proponent of pulling guard. But Ryan Hall from Team Lloyd Irvin is, and in this clip he explains part of his plan in doing so:
11. Armbar from the mount by Royler Gracie:

Paulo Guillobel demonstrates what Royler is talking about in this clip:
And here is clip of a basic variation to capitalize on the opponent pressing you up:
12. Clock choke by "Ze" Mario Sperry:

This is an interesting variation on the Clock Choke using an outside pant grip. Note how his head is still up while applying the choke, this is a common feature of the older variations:
Here is Ze Mario's team mate, Wallid Ismail, putting the Clock Choke on Royce Gracie in a head down variation that is more modern (see the whole match here to see how this position came to be):
13. Half-guard sweep by Roberto "Gordo" Correa:

In lieu of being able to find a better clip, here's my team mate Ben getting me with a deep half guard sweep at the 2007 Arnold's:
14. Take down by Ronaldo "Jacare" Souza:

Here's a drop knee version of the kata guruma from a judo match:
Here's a standing version featuring Judoka Josh Resnick:
And, here's a Judo Highlight vid to inspire you to train your throws:
15. Choke from the mount by Leonardo "Leozinho" Vieira:

A little clip from the old "In Action" tapes of Rorion Gracie using this choke 3 times in a row to beat the same Hapkido guy in some challenge matches. He's mainly using the one palm up - one palm down variation. One thing I had never noticed before now is how much he arches his back when applying the choke.
16. Choke from the guard by Pedro Valente:

“There are some variations to this move, but the traditional grip is made with both thumbs facing the outside. As they achieve the grip, the fighter must twist his wrists (palms up) and pull the foe’s head toward their own chest. It is important to bring the opponent’s head very close to the chest, stopping them from defending or counter-attacking with punches. The pressure must be exerted continuously until the adversary gives up or passes out, which won’t take long if the technique is properly applied. When they use this choke the fighter must pay attention to the guard pass. In order to avoid the pass, they must use their legs, for their hands will be busy attacking.”
This is a clip of the most basic variation of this choke:
Jean Jacques Machado shows a more advanced variation:
17. Sweep from the open guard by Vinicius "Draculino" Magalhaes:

Draculino is referring to a sweep from an open guard variation more commonly known as the de la Riva (DLR) Guard (named after Ricardo de la Riva Goded). Here's a clip of de la Riva showing a sweep from a variation of the guard he made famous: (Kinda funny - he also calls the outside leg hook the "de la Riva" hook.)
18. Ezequiel by Marcio Feitosa:

Here, Judoka Hidehiko Yoshida uses the Ezequiel (Sode Guruma Jime) to tap out Kiyoshi Tamura in a Pride MMA match:
(*Trivia: The Ezequiel choke (sode guruma jime) is the other technique named after a Judoka in BJJ. It is named after a Brazilian Judoka, who had much success with it when he would enter BJJ tournaments. Unfortunately, I haven't been able to find out what his last name was.)
19. Knee on the belly by Gabriel "Napão" Gonzaga:

A little knee on belly with a baseball bat choke to top things off. It's in French, but you don't need audio:
20. Leg lock by Eduardo Telles:

This is a nice basic clip that describes what Telles is talking about:
So, there they are - the Top 20. I'm interested to know what your opinions are and if there are techniques that you feel should be added to this list of 20.