This is my first guide.
I just bought Singed, and his Passive, Empowered Bulwark, surprised me.
I had been playing with Volibear with quite some time and enjoyed his synergy between Warmog's Armor, Atma's Impaler, and Frenzy .
But I saw that Singed had the ability to bypass even Volibear.
This build is fun, but not the MOST effective.
And so we move on!

Pros
Massive survivability
Incredibly powerful slow and throw
With Insanity Potion and Poison Trail active you have a very powerful escape
Generally hilariously amazing to play!

Cons
Weak early game
Gankable until level 6
Not a high damage dealer
Reliant on teamates to do damage
Need good farm to do well

The 3 Greater Mark of Mana are there for Singed to gain some early game health, while the 6 Greater Mark of Knowledge help him snow-ball his Mana, and through his mana his health and his attack damage. The 9 Greater Seal of Knowledge are here to preform the same function, as are the 3 Greater Quintessence of Knowledge.
The 9 Greater Glyph of Scaling Ability Power are here to provide Singed with some extra Magic Resist, as this item build is much heavier on Armour than Magic Resist.

I have the Masteries set up as 0-21-9.

With this build, Singed doesn't need anything in the offensive tree as his Synergy will more than make up for it.
The 21 in Defence provides Singed with the standard Tank ability: to take damage.
The health Masteries within the tree will also help Singed with his Synergy even more.
The 9 in Utility allows Singed to enjoy a faster recall, along with more Mana and more speed.
Combined with the 3% speed gained from Initiator , as long as Singed is above 70% health he moves at a 7% increased movement speed. This allows him to get into and out of fights much faster.

With League of Legends' current Meta-Game, Flash is pretty much vital. The ability to either close a gap or make one larger is instrumental to your success.
Your second Summoner Spell allows for some flexibility. I picked Heal for survivability, and being able to heal nearby team mates who are under fire.
Alternatively, Cleanse and Ignite are both solid choices.
Cleanse allows you to escape, as you don't have many options to stun closing enemies, besides slowing them. Cleanse also allows you to chase, breaking through any stuns that are presented before you.
Ignite is also a solid choice. It gives Singed the early game finishing power he needs if he is presented with an aggressive foe. Beyond that, its uses are limited.

This is the part where a bunch of players will scratch their heads.
You start with the Sapphire Crystal, as it will give you slightly more health.
With your runes and masteries, you should have a fair amount of mana, armour and magic resist, even early on.
Take the 2 Health Potion, just in case.
Recall when you have enough for Catalyst the Protector and a pair of Boots. The extra speed will allow Singed to dodge more attacks, while the health gained from his passive and the item will increase his laning presence.
Recall when you can buy the Rod of Ages and afford the Ionian Boots of Lucidity. The boots may be a strange choice, and they are. With all of this mana, Singed needs somewhere to put it! So with the Cool-down Reduction granted by the Ionian Boots of Lucidity, you can Fling and Mega Adhesive even more! Alternativly, you could switch these boots out for the Mercury Treads, as the extra Magic Resist and the Tenacity help immensely. It's up to you. Rod of Ages works great with Singed, and even better with this build. The Ability Power is nice, but the real good stuff lies in the mana and health.
After this, rush a Manamune. This is where your synergy starts to kick in. Your passive, Empowered Bulwark will give you health for your mana, while your mana will give you Attack Damage.
By now you'll notice that you're a little squishy. Recall and buy the Glacial Shroud, or if you can afford it, the Frozen Heart.
The Frozen Heart is amazing for Singed. Not only does it provide him with armour for survivability, it provides him with mana, which is now granting bonus Attack Damage and health.
At this point in the game your mana should be quite high. Pump it even higher with a Banshee's Veil. The Banshee's Veil will grant Singed more health, which will soon be extremely advantageous. It also grants more mana, which in turn grants... You guessed it! More health and Attack Damage.
The final item, Atma's Impaler, is where everything comes together. The critical strike bonus works well with your growing Attack Damage, and the armour is quite nice for more survivability, but these aren't the reasons you want Atma's Impaler.
Remember all that Mana you've been saving up? Remember that bonus health from Rod of Ages and Banshee's Veil? Now not only does your mana contribute to your attack damage, but your health does as well.

So it goes something like this: Mana goes into health and attack damage, while health (from items and your passive) goes into attack damage. You are also quite tanky, and your Cooldown reduction is good enough to allow you to support your team even more.

At the beginning of the game, you'll want Solo top lane. Take Fling in order to deal with enemy champions of they get to close. Don't push the minion wave, as you want to be fighting near your own turret until you gain more strength. If your adversary gets aggressive, Fling him/her into the turret for some free damage.
Next you'll want Poison Trail. Poison Trail allows you to shape the movement and flow of the lane however you please. Take two ranks quickly in order to deal enough damage to make it viable. As with before, don't venture too far out of your turret area, as your escape isn't the best.
At level four, grab Mega Adhesive. This is your bread and butter. Singed cannot do much damage with this build early on, but this is where you signal your Jungler to gank. You Mega Adhesive, Fling, Poison Trail, and get the kill. The reason you didn't pick it up earlier is you don't have the strength to deal enough damage to make it viable.
As expected, take Insanity Potion at levels 6, 11, and 16.

This is the biggest problem you'll face.
Without items, you're very weak with this build. You need a reliable farm, and that's tough to do. Your Poison Trail will help, but more often than not your minions will steal the kill.
I recommend being careful, especially early game. As you are in melee range, and you aren't doing tons of damage, you'll need to move in and out to secure your last hits. Once you start utilizing Singed's Synergy, it will become easier.
Your basic attacks will hit harder, and Rod of Ages will allow your Poison Trail to become affective.

You'll be your teams Tank or Off-Tank. With Mega Adhesive, you gain a powerful slow that's great in team fights.
In addition, Fling is great at taking down escaping foes. Once you hit Fling, activate Poison Trail immediately. This will increase your damage, perhaps enough to secure the kill. Insanity Potion also grants you bonus movement speed, health regen, mana regen, armour, magic resist, attack damage and ability power.
All these things come into great use when chasing an opponent, engaging in a team fight, or fleeing for your life.

You may be thinking: "Why the HE double hockey stick would I do this?"
Answer: Fun!
You gain a significant amount of Armour and Health with this build. Your Mana and Cool Down Reduction allow you to use your Abilities with reckless abandon. You can slow enemies. You can trap enemies. You can throw enemies.