Not Updated For Current Season
For this build, you will harass early on. if the opposing lane is far too heavy or invis, you may also hug, thats not a problem.
Rune reasoning: Singed's passive per the mana runes and mana/level runes. Health regen is always useful in a tank. Attack speed for taking towers with Insanity potion and first blood.
First blood: Walk people home. When they are on the last 1 or 2 bars, ghost towards their tower and turn on poison. Walk on them and peace out via the side exit. Aim for double kills. Your death is worth 2 kills, one being a first blood.
The rest: Don't back down.
Take poison first. Don't take Adhesive unless the enemy team has a squishy that can give you easy kills.
Insanity, Poison, Fling, Goo (hi to lo priority)
Follow the item orders as listed but if you can buy pieces, always do. The expensive ones being1st in line.
Rush 2x rod of ages. The first one you can obtain at 12~ mins. The second one at 23-25. Rod of Ages was made for Singed's passive.
Ghost, your quints, and masteries down the defensive tree will help your movement early on.
Grab basic boots and don't upgrade unless you need something like Mercury's Treads.
Grab the Guise to make you more aggressive and keep you regenning, followed by Force of Nature (At this point you should have 3.8k HP). Finally Spirit Visage.
Note: Sometimes I switch up the above order if I am wealthy. This is because Force of Nature also improves movement, greatly.
Now you have 2.5k~ mana and nearly 4k HP. With Guise and Force of Nature, this makes you invincible. If you face someone whom takes Executioner's Calling; switch to Guardian Angel/Frozen Heart.
I always end up with no more than 3-4 deaths and the most assists in the team.
Per DEWO's note; in a game where you are playing against only one DPS, you should be more supportive. Consider: Switching Rylai into the second RoA and picking up Arch Angel's Staff instead of Visage (further exploiting the passive and the fact that you will be slowing people down via adhesive and flinging much, MUCH more.This will give you around 4.1k HP towards the end game if you play aggressively, also assisting Force of Nature. On an additional note: Arch Angel's staff being switched for visage gives more mana regen.
One last note for every Singed user: DO NOT FORGET TO USE THE ADHESIVE BOTH DEFENSIVELY AND OFFENSIVELY, THE SAME APPLIES FOR YOUR FLING. DURING CHASES YOUR POISON TRAIL IS ALSO A GREAT BENEFIT TO THE TEAM.
Ideal lane partners: Ezrael (Speed and healing for tower diving early on using Ghost), Soraka, Twitch, Evelyn.
My apologies for not editing and posting currently. :)
-- May 17/2010: Masteries changed.