/back
/ban
/ban-ip (You can put the persons name and it will ban their IP)
/bank
/bank-balance
/bank-create
/bank-pay
/bank-set
/banlist
/buy (No blocks, only items)
/clear
/debug
/defaultgamemode
/deop
/difficulty
/dragonnuke
/enchant
/explode
/feed
/fireworks
/fog
/gamemode
/gamerule
/give
/god
/heal
/help
/home
/kick
/kill
/kittycannon
/list
/me
/ninja (Its not clear in the name, but this creates a controlled explosion)
/op
/pardon
/pardon-ip
/prefix
/pvp
/save-all
/save-off
/save-on
/say
/seed
/sell
/sethome
/setwarp
/spawn
/spawnpoint
/stop
/tell
/time
/toggledownfall
/tp
/weather
/tpa
/unban
/unbanip
/warp
/weather
/whitelist
/whois
/xp
For non-Op:

/help
/me
/seed
/sethome