This is my Nocturne build and I am glad that you have took the time to read this. This is my first (good) build, so I hope that you like it.
The runes I use are for magic penetration, attack speed, attack damage, critical damage and magic resist (just in case). The reason I use a wide variety of runes is because Nocturne needs all these to make up a strong build.
I use flash because Nocturne can get into some pretty sticky situations and might need to flash out, use Duskbringer and escape with the small boost of speed from said spell. Ignite I use for purely attacking champions, giving me that edge over them when in combat.
A good combo to use is Duskbringer, Unspeakable Horror, then use ignite while Unspeakable Horror is going and when they are feared, attack them wildly and if they are still alive and try to run/slow you, use flash then Duskbringer again, and attack them some more. Almost has never failed for me. Guaranteed by me ;)

Early game I buy Doran's Blade because of the extra attack damage and small amount of life steal.
Harass the other team with duskbringer, and farm minions for a while. If an enemy player comes near you or your team mate, use Unspeakable Horror on them, ignite them and use Duskbringer for an early kill. the other enemy player SHOULD back off, but if they don't, you and your team mate attack them while moving back, drawing them out. When they have realised their mistake, it will be too late. I'm not saying this exact scenario will happen, I'm just saying that it usually happens to me (depending on who I'm versing). I try to lane with someone who has exhaust and is a ranged character most of the time. Some exceptions are Mundo, Olaf and Udyr, because their stun/slow can help a lot during fights.
I buy Berserker's Greaves around now for attack speed and movement speed and if possible buy some health and mana pots, just in case.
Back in your lane, do the same thing again as you did when you started (harass, farm minions etc) for a while until someone destroys a tower in you lane.
Buy a Vampiric Sceptre for some life extra life steal, and lead it on to a Bloodthirster. Now as you farm, you will gain attack damage when you kill a minion or anything. Be careful not to die though, or you'll lose the extra damage and life steal and will have to start farming all over again to get back the extra attack damage and life steal.

This is around about the time when everyone has left their lanes and will start ganking more often. I like to wander around the map ganking. Ganking is much easier with Nocturne's ultmate, especially if there is a team fighht in mid lane, you can really turn the tables by ulting in, using duskbringer etc. build up your Infinity Edge, gaining critical strike and more damage. Farm some more in which ever lane you started in for a while until you build up enough gold to get Phage, then build it up to Frozen Mallet. This gives you some health instead of being squishy, and an easy target. It will be much easier to chase down champions now to, because you can use your ultimate, hit them so they get slowed and use that combo I mentioned in 'Early Game'.

Now that your quite far into the game, start building up a Phantom Dancer by buying Zeal, giving you some extra movement speed. This gives you some leverage when your flash/ultimate is on CD and they are getting away (the enemy champion). Sell your Dorans Blade, and start building up your Hextech Gunblade. Push mid with your team until you get to the inhibitor, take out some champions and win the game :D
I hope this build helps you out, and if you have any questions, just ask. (Remember this is my first (good) build, so please don't be to hasty to judge ;D )