The Omni Plant is a special plant in The Sims 3 that can be used to replicate many other plants, as well as other objects -- including some books and fish. The plant can only be planted by Sims that have a gardening skill level of 10. Some special items, such as death flowers or the Ambrosia recipe book (worth 12,000 simoleons), cannot be replicated using the Omni Plant. The Omni Plant can die if you do not water it, but it takes a very long time.
The Omni Plant is obtained by completing the Uncommonly Good, Outstandingly Rare, and The Omnificient Plant opportunities. It may also be found at the Landgraab Industries Science Facility or the Llama Memorial Stadium, while fishing, or received as a random reward after going into the past in the time machine.
The Omni Plant is excellent for getting rare fruit and fish, because it can be fed with anything from life fruit to robot fish and deathfish. The Omni Plant can also be fed with books or teddy bears.
Once you have received the Omni Plant for the first time after completing the opportunity, you can order more seeds through the mail.