Hecarim, the Centaur faster than his own shadow.

High speed does not only mean high mobility and great ganking/initiator - with Hecarim, it's much more than that. Having high speed means a vast deal of damage. Combined with Rampage it allows Hecarim to deal tons of damage, especially if combined with Ghost/Devastating Charge/Youmuu's Ghhostblade.
With this build, no one outruns Hecarim, and it's possible to surprise any loner and make them shortlived, just to disappear again moments later.

With his 320 basic movementspeed, and 9.5% speed from masteries and runes, it's possible to get more than 900 movementspeed when initiating a fight with his item, spell and skill. That's 580 bonus movementspeed, which equals to 145 bonus damage on level 18 from his passive.
Now, that's alot of damage, and with a stacked Rampage you'll finish of a fairly weak support/glasscannon before they've much time to reach.
Follow up with your ulti, and your team will be in the fight to take advantage of the surprise. Depending on the situation, run back a bit to get clear of their targeting, or continue to wreck havoc in their midst.

Skills
Start with Rampage, follow up with Spirit of Dread and then Devastating Charge.
Depending on how much damage you end up taking, continue to max Rampage and Spirit of Dread, always have them on equal ranks and the one you feel like you need the most to skill up first. And of course, spec in Onslaught of Shadows whenever possible.
Finish with maxing out Devastating Charge.

Runes
In my build I go with Armor Penetration to get the most out of spamming Rampage, as for blue and yellow runes, I fill them with armor and magic res to get that needed survivability.
Quintences gets the 4.5% movementspeed for obvious reasons. Speed is of the Essence.

Masteries
I tried a couple of different builds before deciding on this one. 4/17/9
It gives Hecarim the most possible out of jungeling, it also gives him a bit better sustain than the normal AD-build, while still allowing Hecarim to deal a fair deal of damage - and as always, Speed is of the Essence.

Spells
This one is a no-brainer. Smite is essential for jungeling, and Ghost gives mobility as well as a damage burst for Hecarim.

Play Video
This is one way to do the jungle.
It's also possible to do the noobie-standard Blue > Wolves > Imps > Red > Wolves. Then go to the giants when smite is available.

The most important part about Hecarim is ganking. Hecarim is a great ganker and should fish for ganks whenever he see an opening. Don't be afraid to waste Onslaught of Shadows, use it whenever you see fit.
When ganking, always try to get a good angle. Run back around the enemy to get the knockback towards your own tower to allow a few more hits from you and your teammates. Same goes with the fear from Onslaught of Shadows.

Hecarim is a hero who scales exponentionally with his items. The faster you can get an item, the more advantage you will get. Now, this is true to all heroes, but for Hecarim it especially so thanks to his damage and speed scaling.

As with all heroes there's Pros and Cons with all builds.

* Hecarim is depandent on getting a good farm and ganking-tour early on. If he fails to do so, he gets behind and will have problems keeping up with everyone else.

* It's all about situational awareness. With such high mobility it's important to analyse the situation before rushing into battle. Is your teammates close enough to follow up on your charge? Will you be able to finish off their weak champion before they can finish off you? Can you get out of there with your speed?

* As this build is somewhat squishy, stuns and such crowd controls are Hecarims worst nightmare. You don't want to get caught without being able to move. Moving at high speed is Hecarims greatest weapon, disable that and he's just another Centaur.

* Hecarim can solo the dragon as soon as he has Wriggle's Lantern. Have your bot teammates put a ward at the bottom of the river, and put your Wriggle-ward just above the dragon.

* Hecarim is a great ganker and initiator. He's the hit and run beast.
Use him to initiate fights and ganks, always try to push the enemy towards your teammates with Devastating Charge. Also try to finish off their weak targets as soon as possible to get an advantage in numbers.

* With Wriggle's Lantern, Force of Nature and Spirit of Dread, Hecarim gets great sustainability. He can run around jungeling until he gets bored, or push a lane beyond infinity without having to consider anything but the enemy champions.