Effect (player) (effect) (time) (strength)
The potion effect strength can go as high as 250 but by then it usually cancels itself out, so control MATTERS!
4 Mining Fatigue
6 Instant Health
7 Instant Damage
8 Jump Boost
12 Fire Resistance
13 Water Breathing
16 Night Vision
21 Health Boost
If you have a server, such as RPG or fighting, or arcade,
you can use command blocks (ID 137) to give people potion effects for certain situations.
If you have cheats enabled you can obtain command blocks with the following: /give (player) 137
More on this complicated topic:
Thanks to simonh2000 for putting this together!