I have looked and can't seem to find any.
I have seen many green cards and some artifacts that do this, but nothing in Red, Blue, Black or White.
Are there any?
Note: By "Mana Ramp" I mean a card that generates mana, or one that lets you play more than one land per turn. The end result, for example, would be on turn 4, playing a white only deck, I could tap more than 4 mana. (Normally you only get one land per turn so a max of 4 mana on turn 4.)
There are, but they are purposefully few and far between.
Mana acceleration is considered to be in green's part of the "color pie," with the exception of the occasional black card, and a little bit of a fast-mana theme in red (formerly black). Blue and white occasionally get ways to cheat on mana a bit, but only when they're tied into some other element of those colors.
This answer has discussion tailored to Commander, which also applies pretty well to other casual and multiplayer decks.
In competitive terms, the most relevant cards are:
Note that there are a lot of colorless cards that are used to cast normal colored cards (like Ancient Tomb).
Also note that oftentimes it's more effective to "cheat" on costs (using reanimation or hand-to-play cards like Show and Tell) than to accelerate your mana.
Mana ramp is definitely a very green thing; it's a natural effect for the color of life and growth (among other things). This is a great example of the color pie at work: there are just some things that not all the colors can do. But there are some cards outside green (and artifacts) that help produce mana.
In other colors, mana producing abilities are often more conditional - maybe it's a one-time effect, or maybe you have to pay something else to get it.
Here's a Gatherer search that'll find (hopefully) all the directly mana-producing options (along with some things that don't actually ramp, like ones that just turn one color into another). The same search restricted to Standard only finds six cards, only three of which are repeatable: Crypt Ghast, Deranged Assistant, and Liliana of the Dark Realms. (Liliana isn't exactly speedy ramp, of course.) You could restrict to permanents if you don't want one-offs.
It's hard to search directly, but there are also non-green nonartifact things which help put lands onto the battlefield. Here's a deliberately inclusive search; most of those are false positives, but there are a few cards in there that put lands onto the battlefield for you: Braids, Conjurer Adept, Dreamscape Artist, Firebrand Ranger, Knight of the White Orchid, and so on. Again, they're more conditional - and this time, none of them are in standard. (And some of them are from Planar Chaos, where the colors were a bit wonky.)
White has very little ramp of its own; the effects it does have relating to mana are generally dependent on you controlling fewer lands than your opponent, and most of those put land in your hand, rather than on the field (which is not ramp). Examples: Land Tax, Kor Cartographer, Knight of the White Orchid, Oath of Lieges.
Blue also has very little. Dreamscape Artist is a spellshaper that knows a color-shifted version of Harrow, there's the infamous High Tide, and most (all?) of the "free mana" cards from Urza block are blue, such as Palinchron. Don't forget twiddling effects that can hit lands, like Turnabout!
Black has enough ramp to spawn an archetype -- Big Mana Black! Between cards like Cabal Coffers, Nirkana Revenant, Crypt Ghast, Liliana of the Dark Realms (ultimate), Skirge Familiar, and rituals along the lines of Dark Ritual, a black deck can achieve absurd amounts of mana.
Devotion If your deck is a devotion deck Nykthos, Shrine to Nyx is a powerful ramp card.
While older black cards included many rituals, modern rituals tend to be Red. See: Battle Hymn, Brightstone Ritual, Desperate Ritual, Infernal Plunge, Koth of the Hammer (first two abilities), Mana Geyser, Pyretic Ritual, Seething Song, etc.
Terrain Generator is a land that not only can add colorless mana but if you have extra land at the end of your opponents turn it allows you to put a basic into play
Why does nobody list Sisters of the Flame (1RR). Taps for a red mana. A 4th Edition & The Dark card.
There's a few, but they aren't very common, and most aren't repeatable effects. I won't list what everyone has already said again, but there are a few I haven't seen mentioned that probably should be:
Another way to get more than one lands into play in a single turn is to recur lands (especially fetchlands) from your graveyard. White in particular uses this strategy since it has some decent cards capable of bringing back permanents and lacks some other more direct ways to ramp.
Cards like Second Sunrise, Brought Back, and Faith's Reward can interact with fetchlands (e.g. Arid Mesa) that were sacrificed to put them back onto the battlefield for a second use. Or more direct permanent recursion like Sun Titan can also return lands to the battlefield and act as mana ramp.
There'd Braid of Fire, but that's only upkeep.
You can also take control of your opponents' lands such as with Annex, Agent of Treachery or Conquer. Chandra, Novice Pyromancer can give you RR for a loyalty counter. Combine it with Chandra's Regulator and Chandra, Acolyte of Flame and you can get RRR each turn.
There's also cards that take you extra turns, which is mana acceleration in some sense.
Blue has a minor theme styled as individuals creating mana to cast (in-world) spells. This tends to manifest in a way that interacts with the card's set's flavor.
Some examples:
