This is the way that I like to tank with Singed. I prefer being able to move and cause havoc across the map with ease. I like being able to soak up a lot of damage while also being able to cause considerable damage to your enemies.

Using items like Randuin's Omen and Rylai's Scepter, as well as your Mega Adhesive ability, gives you effective slowing capabilities to control and disrupt your enemies.
Insanity Potion, and summoner skills like Teleport and Ghost give you the ability to travel across the map with ease.
Poison Trail and Fling allow you to be a threat when combined with AP items as you are able to mix magic and physical damage to affect the area of your team fight and protect your allies.
I think this is what makes an effective tank, and what makes Singed my favorite tank to play.

Post your scores and I will post the top 5 best scores (by K/D ratio, 0's will count as a 1.)

Top Scores:

Pros:
- Great initiator
- Can soak a lot of damage
- Slows with Mega Adhesive, Randuin's Omen, and Poison Trail (with Rylai's) gives him great CC
- Increasing movement with his Ultimate, Ghost, and being able to Teleport make him versatile
- Getting items like RoA, Abyssal Scepter, and Rylai's give him great damage as well as great tanking potential.

Cons:
- Fling can be difficult to master/may get your teammates mad.
- Requires a good amount of practice to excel
- Sometimes inefficient with Mana
- Weak early game. Play with care until level 5/6.

Magic Penetration Marks - These are going to give you more poison trail damage, making you a problem for most enemy champions.
Mana Regen Seals - These will allow you to have more ability uses before needing to go back to base.
Magic Resist Glyphs - These will let you take less damage from any magic damage sources.
Health Quints - Having more health will keep you in the fight longer, and keep you from running away when you're needed by your team to stay.

That being said, you can use whatever runes you're comfortable with.

Teleport + Ghost are my favorite summoner spell combination on Singed.

Other acceptable spells could be:

Exhaust - This is useful on any auto-attacking champions, and a great summoner spell to use with any Tank character who will be in the midst of a team fight.
Ignite - I prefer this less on non-dps characters, but you can do some real damage with your poison and ignite.
Cleanse - This spell is useful on any tank champion, or any champion to expect to be the target of heavy CC.
Clairvoyance - Only if you're talented with it.

Other summoner spells aren't really worth mentioning.

[R] > [Q] > [E] > [W]
Insanity Potion > Poison Trail > Fling > Mega Adhesive

You can take Poison Trail at Level 1. I choose to take Fling for possible First Bloods. Sometimes enemies will initiate you by your tower early on allowing an early fling for a kill.

Rod of Ages is the best item to start with because of how tanky it makes you while giving you a good mana pool to work with. The fact that it gets better over time makes it invaluable.

I would take Abyssal Scepter over Banshee's Veil because of getting AP along with your magic resist. The fact that it lowers the magic resist of all nearby enemy champions helps all of your casters and your poison trail to do as much damage as possible.

Rylai's gives you more health, more ap, and gives your poison trail and mega adhesive more slowing capabilities. It is pure win.

The two interchangeable items in this list are Randuin's Omen and Frozen Heart (As pointed out in the comments over Guardian Angel for mana, armor, and more useful passive).

If you need more Mana and AP I would get a 2nd Rod of Ages.
If you need more Mana and Magic Resist I would get a Banshee's Veil.
If there are too many auto-attackers, get a Thornmail.
If you are really blowing the enemies out of the water, grab Rabadon's Deathcap.

AP/Armor/Magic Resist/Health+Mana items are all useful on Singed. Always pay attention to each game to understand what items you need to build!

If you want to switch swiftness boots out for merc treads, do so only against very high crowd control/magic damage teams as the increased speed is useful chasing down enemies to catch them in mega adhesive or fling them back to your allied DPS.

USE POISON TRAIL TO FARM!
You can level up faster than the champion that is in mid. You can use teleport to go to open lanes, or even run through the jungle and let poison trail do the work. After your Rod of Ages is charged and you have either Abyssal Scepter or Rylai's, you should be able to take down waves of minions quickly.

It is important, but not completely necessary, to have the blue buff with Singed. It helps his mana problems a lot. Blue buff also allows you to farm more by running around with your Poison Trail and giving you CDR.

If you use teleport you should be fine to go back really quick while your team stalls any enemies and jump right back in the action.

1) Be sure to fling characters like Warwick, Tryndamere, or Master Yi away from your squishy characters. Fling can save lives and give your DPS enough time to kill large-threat champions. Fling is also useful to put enemies that are almost in tower range into tower range for high damage.

2) Use Mega Adhesive to keep the enemy team separated, or to slow them from getting to your ranged/casters. Don't forget to use it on fleeing enemies. It can also deter enemies from attacking your tower, along with a cloud of poison.

3) Ghost + Ultimate + Poison Trail whenever entering a team fight. If one is on CD try to use as many as possible.

4) Don't be afraid to initiate! This is Singed's greatest strength IMO. He can be beat on by all 5 enemy champions and get away alright. Slows, Poisons, and Allied Champions will make the enemies sorry for sticking around!

Whether it be + or - votes, please give an explanation or comment as to why you voted.
If you think I need more clarification in any part of the guide or would like to see other chapters added, please feel free to ask and I will work on it.
Thanks everyone!