Ketamine. Ketamine offers very interesting possibilities for depressed patients who don't respond to standard therapies. It does induce dissociation and sometimes hallucinations, but in some there's an antidepressant effect that lasts for several hours/days after those are gone. Some drawbacks: can be liver damage; it must be given IV in a controlled setting; effects may wear off. All is being studied.
We don't know yet. Although ketamine has gotten press recently surrounding some studies which have shown a significant benefit in the setting of acute depressive episodes, there have yet to be adequate studies supporting its use as an anti-depressant. There are significant safety concerns regarding ketamine, including perceptual changes and hallucinations. At high doses, it induces general anesthesia.