Bukkit

Create custom potions

1: get potion (id=373) to your hands (can take potion with any data 373:1 373:2 etc).

2: (Required) command
/nbt item CustomPotionEffects[0].Id = X byte
Where X - Effect ID. see http://www.minecraftwiki.net/wiki/Potion_effects

3: (Optional) command
/nbt item CustomPotionEffects[0].Duration = X int
X - duration in ticks (0 ticks by default)

4: (Optional) command
/nbt item CustomPotionEffects[0].Amplifier = X byte
X - amplifier of the effect, with 0 being level 1 (0 by default)
you can repeat points 2-4 to add one more effect (CustomPotionEffects[1])

5: Rename your potion!
/nbt item display.Name = "My own potion"

see http://www.minecraftwiki.net/wiki/Player.dat_Format#Potion_Effects