If you're like me, you traded for a Candelabra of Tawnos 6 months ago at $25 and just traded it away at $175. This didn't happen because I predicted the unbanning of Time Spiral and subsequent reemergence of High Tide as a deck. This happened because I play EDH (Commander). Candelabra is a good card that works well in EDH, so I had one when the price spiked. I'm not saying I'm about to make you hundreds of dollars, but there are certain things you should be looking for if you play EDH so that you can reap the benefits of spikes like this.
Before going into further detail, I should also mention that it's almost certain all EDH staples are going to increase in value this summer when both the Commander precon decks and From The Vault: Legends are released. These two products should generate interest in the format, which means demand will increase. Even if you don't pick up anything that has a ridiculous spike, there's still a high likelihood of a lot of these cards gaining value. How much I can't tell you, but most of the things you're looking for are so cheap it's almost impossible to lose money anyway. Here are the categories of cards you should be looking for:
Foil Legendary Creatures - This one is obvious, but it still bears repeating. Legendary creatures are on the rise, and a lot of them are far too inexpensive for how powerful they are as generals (commanders).
Colorless Utility Cards - This is another obvious one, but I don't want people in the comments saying that I ignored either of these groups. Colorless utility cards are the cards that can go in every deck. Many of these have already seen EDH related price hikes, such as Maze of Ith and Sword of Fire and Ice and its brethren. Some already have price hikes to the foils like Mind's Eye, Sculpting Steel, and Oblivion Stone. Finding cheap foil EDH cards is certainly the best investment, but even non-foils have a good chance of gaining value, albeit less dramatically so.
Along these lines, I absolutely recommend picking up Contagion Engine every time you get a chance, especially foils. This is THE colorless utility card: it wipes an opponent's board, helps you kill people with poison, pumps any of your creatures with +1/+1 counters, and accelerates your planeswalkers. It even has more bizarre and hilarious applications like making Goblin Bomb threatening or Chance Encounter a viable win condition.
Shards Block Foil Mythics - A number of these mythics are legendary creatures, so technically they're already covered. The reason that I'm singling them out, however is the all foil Shards block packs. It's no mystery that the older a card gets, the more its price will rise if it sees continuous demand. Most old cards are worthless, but the cards that have always had demand, such as Sliver Queen and the Darksteel swords, keep going up (no promises as to how quickly). Many of these mythic rares will see continuous demand because they're great for EDH, be they generals or otherwise. Most of these mythics are also grotesquely undervalued because of the glut of foils resulting from the ill-conceived Shards block foil packs. Kresh, the Bloodbraided foil can be picked up for about $3 pretty easily. Considering that this is one of the easiest generals to kill someone with in a single hit, that's pretty strange. You can get Maelstrom Nexus for about the same price, and that's a must-have for every five color EDH deck ever. There are still foil Shards block packs available because the demand for the packs was so low (which, if you're reading, Wizards, was only because of the price) but once the influx of new supply dries up, we'll see the price rise. I expect some of these cards to have dramatic increases.
Cards With Off-Color Abilities - People enjoy playing mono-coloured EDH decks for a number of reasons. You'll never get colour screwed, you don't have to spend money on expensive mana bases, and you get access to powerful cards like Extraplanar Lens and the [card Sapphire Medallion]Tempest medallions[/card]. Every color has its weakness, however, because of the colour pie. Of course, the colour pie has changed multiple times, and Wizards used to frequently prints off-color cards.
Have you ever lost a game to Sword of Light and Shadow because your mono-black deck can't destroy artifacts?
Well a card that has seen a recent price spike from mere pennies to multiple dollars, Gate to Phyrexia, can keep that from happening. These cards are very limited in how much they can raise in price because they're normally very specific and often underpowered cards, but any profit is good. Oh, and making your deck better is good too.
Cards With Obviously Powerful Abilities That Nobody Cares About- And here we have it. This is where you have the chance to make real money, if you get lucky. Candelabra was a blatantly powerful card. In a format like EDH where lands [card Miraris Wake]never tap for a single mana[/card], Candelabra was like a 1 mana Mirari's Wake. Eureka has an enormously powerful ability that was great for multiplayer fun. And I'm not even going to get started on how many stupid applications Power Artifact has. Now many of these cards that no one cards about, cards like Karmic Justice, Mana Reflection, and Hunting Grounds are unlikely to ever have tournament applications. Flip through your binders though or search through old sets on gatherer to find more cards with ridiculous abilities. If they're affordable and have EDH applications, there's a good chance you'll see them rise at least a little bit. And if you get really lucky, you may have just accidentally bought the next big legacy card.