In Commander, it's typical for decks of all colors to run "mana rocks" -- artifacts that generate mana. Some of the popular ones are: Sol Ring, Mind Stone, Thran Dynamo, Darksteel Ingot, Worn Powerstone, Dreamstone Hedron, Mana Vault, Mana Crypt. Some of these help with mana-fixing, others just produce colorless mana; the former is often helpful in a multi-color deck, but for mono-color you can just go with whatever's got the best resource yield. These cards are less sexy than straight-up mana doublers, but they'll give you a consistent edge right out the gate -- and usually draw less hate, to boot. (Spectral Searchlight is a pet card of mine I'd like to throw in here. It lets you play "friendly" by fixing someone else's mana or letting them cast some big spell a turn early, if you wish.)
Beyond mana rocks, any color deck can use Solemn Simulacrum, Pilgrim's Eye, Expedition Map (use it to tutor for cheaty lands like Coffers), and Doubling Cube. You can play mana Myrs if you seriously need the extra juice, but be ready for them to die to board wipes (Plague Myr deserves special notice -- slap some big equipment on him and he turns into a major threat).
There are colored cards that can do ramp, too. While green is known for ramping, being able to access almost the entire Vintage card pool (and getting to play higher-cost stuff because the game's not over in just a couple of turns) means every color has access to its own brand of acceleration.
- Black's acceleration is surprisingly strong in the format. Cabal Coffers is very popular in black decks (or multi-color decks running Urborg, Tomb of Yawgmoth). Magus of the Coffers and Nirkana Revenant give you even more Coffers-like effects. Black also has fast mana with old-school ritual effects like Dark Ritual and Soldevi Adnate.
- White has rather little, but you can get extra lands with cards like Land Tax, Weathered Wayfarer, and Knight of the White Orchid (get your land count lower than the rest of the table by using Ravnica bouncelands). Serra's Sanctum is strong in the right deck.
- Blue's biggest asset is easy card draw. You can put a bunch of mana rocks in your deck without having to worry as much about them messing up your late-game topdecks. Drawing multiple lands per turn also combines nicely with Walking Atlas, Patron of the Moon, and Azusa (if you're blue-green).
- Red has some pretty good fast-mana spells; for Commander, I like Mana Geyser and Brightstone Ritual. As far as more lasting options... Braid of Fire has already been discussed; Koth of the Hammer is solid, but will probably draw a lot of hate; Soulbright Flamekin is a quirky and underestimated dude who can buff your creatures and fix your colors at the same time. Red has something else going for it, though: plentiful land, artifact, and creature destruction spells you can use to knock out other players' key ramp component.
For a red deck, I'd look to artifacts for ramp first and foremost, just for efficiency's sake. Red's in-color rampy stuff is nifty but I find it difficult to use.
If you're looking to generate prodigious amounts of mana to power out X spells, you can also get more bang for your buck with damage doublers like Fire Servant, Quest for Pure Flame, and Furnace of Rath; and copy effects like Reverberate (copied spells have the same X values and kickers as the original).
