It dawned on me that Recurring Nightmare could be one of the most efficient creature creators in the whole MTG universe.
The concept behind Recurring Nightmare is to bring back undead minions that have been sent to the graveyard, again and again...skeletons, zombies, wraiths, vampires, etc. Standard black strategy.
But it occurred to me, there are a couple of other strategies that really make the card shine.
#1) The Haste/Comes Into Play Strategy
Essentially a mono-black deck that uses Buried Alive to put creatures (of any color) with "comes into play" effects directly into your graveyard. Then use RN to put them directly into play from the graveyard, triggering their comes into play effect. You need to destroy your opponent's land...bring in a creature that destroys target land when it comes into play, likewise, cards that force creature destruction, opponent discard, draw cards, destroy enchantments, destroy artifacts, etc. Then sacrifice the more-or-less useless creature to use another Recurring Nightmare effect. Since using the RN puts it back in your hand, and casting it only costs 3 mana...it's a pretty efficient system...just watch out for counterspells.
As much as that strategy is sound, the 2nd kind of deck is more fun. Instead of responding to what your opponent does...make him respond to what YOU do.
#2) Big Nightmare Strategy
For 3 mana (and a creature sacrifice) you can bring ANY creature card from your graveyard, and put them into play. Unlike Raise Dead & the Gravedigger ability, which puts the card into your hand. For 3 mana, I can bring Colossus of Sardia, Phyrexian Colossus, Avatar of Might, Devouring Strossus, Krosan Cloudscraper, Plated Slagwurm, Darksteel Colossus, Denizen of the Deep, or any big elemental (green or blue), demon, angel, dragon, wurm, or sea serpent...big flyers, creatures with trample or vigilance, or whatever DIRECTLY INTO PLAY. If they have a nasty upkeep...just sacrifice them to your next casting of Recurring Nightmare...or not, since your opponent won't last long once these guys get on the battlefield.
As to what "big" creatures to include...go through your collection, dig out the highest casting cost, hardest hitting cards you got. Don't worry too much about upkeep costs, as your opponent won't last long enough for it to matter...but you should avoid creatures that are pump-able in anything but black (ie, dragons that do more damage with red mana...which you don't have) also, be cautious of cards that don't go back to the grave if destroyed, or other effects like islandhome, etc. Look for big creatures with trample, flying, vigilance, shroud, and of course, haste. If they have a good "comes into play" effect, utilize it as well. Up the 'wow' factor by putting some big artifact creatures and multi-color creatures, too. (But avoid the high cost, low power creatures...you know, a 4/4 creature that costs 9 mana).
You will draw some of the big nasties into your hand, so be sure to have plenty of ways to discard your own cards via spell or creature ability. The balance of the deck should focus on sweeping enemy creatures, enchantments & artifacts. Throw in a few equipment artifacts that can further help your incoming horde. Be sure to include plenty of cheap black (disposable) creatures, since you have to sacrifice something to trigger the Recurring Nightmare effect. I like to use creatures with either comes-into-play effects, flyers, or good defenders.
Remember, the magic number is "3"...the cost of RN. When you have 6-9-12 mana generation available (via lands & Dark Ritual), try and use the RN multiple times the same turn. Your opponent will wet themselves.
I just did the test draw available from this site, my opening hand only had one swamp, but I did have a Dark Ritual. On turn 2 I drew a 2nd Dark Ritual. I would use the first one, then with 1 of it's 3 mana, cast the 2nd (5 black mana). I would use 2 to bring the Rotting Rats into play (forcing everyone to discard a card, I discard the big green Engulfing Slagwurm). With the remaining 3 mana, I play Recurring Nightmare. I immediately sacrifice the Rotting Rats, and put the RN back into my hand and put the Slagwurm on the battlefield. Turn 2. One land in play. And my opponent is looking across the field at a 7/7 monster which enables me to gain life when it blocks or is blocked. Not a bad way to start.