Since THS didn't want to make this list, I did for him. I'll update it periodically. If there's a change I've missed, let me know!
Slate cost: 300 + 25% per slate of this type previously bought (ex. your second life slate will cost you 375, while your first life slate will only cost 300).
Level-up: Every requirement increases by 5% per level to make it fair, as the requirement often gets in faster because the effect that is related to the requirement is upgraded too.
Air Slate (Amethyst)
Range: 1000 (Air mobs only)
Level-up: Every 2500 damage done, it levels up. Damage gets increased by 5% of the original amount.
Poison slate (Emerald)
Effect: Applies a poison which hits for 25 dmg/sec while they are on the slate and during 10 seconds after leaving the slate.
Level-up: Every 2500 damage done, it levels up. Damage and poison damage gets increased by 5% of the original amount, plus 0,5 second of the poison's length.
Stun Slate (Peridot)
Effect: Stuns creeps for 1.5 seconds. Effect doesn't stack. Once stunned, creep is immune from this slate.
Level-up: Every 500 damage done, the damage increases by 5% of the original amount and the stun length increases its length by 0,1 second.
Metal slate (Onyx)
Level-up: Every 20000 damage done, the damage increases by 5% of the original amount.
Steam Slate (Aquamarine)
Effect: Slows mobs for 10% of their original speed while they are on the slate.
Level-up: Every 10000 damage done, the damage increases by 5% of the original amount.
Water slate (Sapphire)
Effect: Slows mobs for 20% of their original speed while they are on the slate and during 10 seconds after they leave the slate.
Level-up: Every 5000 damage done, the damage increases by 5% of the original amount.
Fire slate (Garnet)
Effect: Applies burning state to mobs standing on the slate; the effect fades after 10 second. The fire deals 150 dmg/sec, increases speed by 10% and is cancelled by water effects (slowing effects of sapphires and steam slate).
Level-up: Every 20000 damage done, the damage and fire damage increases by 5% of the original amount.
Air-Hold Slate (Amethyst)
Effects: Holds the air mob that passes on the slate for 3 seconds, then waits 1 second before it can hold another mob. It can only hold one mob at a time, and mobs that have been held once are then immune to that slate's effect.
Level-up: Every 40 mobs held, the damage increases by 5% of the original amount and the length of the hold steals 0,1 second of the waiting, so at level 10 there is no waiting. On level 10, the slate may hold two mobs at once, on level 20 in can hold three mobs at once, etc.
Armour-Piercing Slate (Diamond)
Effect: Applies a -12 armour to mobs for 3 seconds and a -3 armour to mobs for the 17 following seconds. The slate's effect needs to wait 5 seconds before being used again.
Level-up: Every 50 mobs affected, the -12 increases by one point and lasts 0,2 seconds more; the -3 increases by 0,2 point and lasts one second more.
Negative Slate (Opal)
Effect: Any creep that passes on the Negative Slate is slowed for 10%, poisoned for 10 dmg/sec and is applied -5 armour. Every effect lasts 10 seconds.
Level-up: Every 100 mobs affected, everything is boosted by 10%
Snare slate. (Peridot)
Effect: Stuns nearly indefinitely, letting a 5% chance to stunt mobs to escape.
Level-up: Every 200 seconds the snare effect is effective, the percentage of a mob to escape is decreased by 0,1% until it reaches 1%, after what the levels will still increase without upgrading anything.
Mine Slate (Ruby)
Effect: One use per round. Damages ground mobs over a medium splash area centered on the slate when triggered by creep.
Level-up: Every 10000 damage done, the damage is increased by 5% of the original amount. (Yes, this one increases barely fast, and it is normal, because it has one use per round)
Massive Slate (Diamond)
Level-up: Every 50000 damage done, the damage is increased by 5% of the original amount.
Magnify Slate (Kunzite)
Effect: Any damage a creep takes while on the Magnify Slate is doubled.
Level-up: Every 20000 damage done by the other towers (only 1x/2x, obviously, I want it to calculate the difference), the damage is increased by 5% of the original amount and the multiplier increases by 0,1x.
Life Slate (Topaz)
Damage: 5% of a mob's maximum HP/sec (at a draining rate)
Effect: Drains health (see Damage)
Level-up: Every 200% of a mob of any level (it will be calculated by percentage, so 5% of a mob on a level and 5% of a mob on the next level will still do 10%), the draining damage will increase by 0,2%.