Well, there are a lot:
Paralysis-Lowers Speed, 25% to not be able to move at all
Frozen-Freezes opponent, random chance of thawing out (not including moves like scald or fire moves)
Sleep-Unable to move for 2-3 turns
Poison-Gradually lowers HP, 12% per turn
Badly Poison-Gradually lowers HP, stacks up per turn
Then there are the "minor" statuses"
Confusion-50% chance of hitting itself
Curse(works only w/ ghosts)-cuts 50% of user's health, takes away 25% of Foe's health each turn
Embargo-Pokemon cannot use items, held or trainer used
Encore-Forced to use moves that it previously used for 3 turns
Flinch-Makes Pokemon unable to move during that turn
Heal Block-Pokemon cannot heal itself for 5 turns
Attraction-Pokemon of opposite gender of user cannot move for 50% of the time, can be recieved through Cute Charm or Attract
Nightmare-Loses 25% health each turn, if sleeping
Particially Trapped-Moves like Magma Storm, Sand tomb, warp, etc. that traps you and does 1/16 of health each turn
Perish Song-All Pokemon that hears this will be perished in 3 turns
Taunt-Cannot use set up moves
Torment-Cannot use two moves in a row
To answer your quesiton, there are about 19 status ailmets that are useful. These ailments, like flinches, attraction, paralyzation, and confusion can be very trolly in competitive battling.
Hoped I helped!