Belladonna and morphine have been used to create a truth serum in the past however it's effect are dubious. The serum effectively renders the recipient unconscious however in the interlude before the reach that state the are less able to lie supposedly. However during that time the are also dissociated from reality so you're just as likely to be regaled about what the fluffy pink unicorn is doing as you are to get any answers, unfortunately this would make any information you received rather unreliable. I would also caution against using either component in a potion as both are toxic and improper dosage causes severe health complications and may be fatal.
Other herbs that have not yet been mentioned that I have heard of being used are licorice root, calamus root (may cause complications if large amounts are ingested) and deer's tongue (advisable not to ingest).