Ignite

As a hardcore player, I find ignite nice to improve your dps while you try to survive. If you calculate the dps thinking you can stand still while shooting fireballs, ignite is indeed a dps loss, but that is not always the case, or you will eventually die.

Pratical example of a Witch I tested, level 55, a 3L item with the following gems: Level 13 Quality 16% fireball, Level 13 Chance to ignite and Level 13 Elemental Proliferation. With the passives and gear, my dps on fireball is 600 and 61% chance to ignite.

When you shoot a fireball, the enemy hit would take 600 points of direct damage and another 800 points of ignite over 4s (which is spread to anyone in a 24 radius due to Elemental Proliferation), mana cost is 38.

You can have a totem down to help with your dps, and walking around refreshing the ignite on the enemies. You can even use Flammability to increase the chance to ignite and lower their fire resistance.

I had no problem so far to kill anything. When I see a pack of monsters, I use Flammability, put a spell totem with incinerate+lmp+firepen, cast the fireball explained above to get ignite rolling, and walk around casting a firestorm every now and then. It is really easy to avoid melee enemies just by kiting them. Ranged is not that hard as well, since most of the time I am moving around and refreshing stuff in this order, totem, ignite, curse.

The chance to ignite gem works well if you have no crit chance. Fireball for example has 6% chance to crit, and with around 90 passives (including defensive nodes), you can get that to around 45% and +150% damage multiplier, so you can change the Chance to Ignite gem for a crit chance gem (around +4% for a level 1 gem no quality). The ignite from a crit fireball is way higher when compared to the flammability+change to ignite one, but you should spend several points in the passive tree.