The following plants are toxic to both cats and dogs (unless otherwise noted) along with clinical signs of poisoning:
• Aloe Vera: Vomiting, depression, diarrhea, anorexia, tremors, change in urine color.
• Asian Lily: Vomiting, inappetence, lethargy, kidney failure, and death is possible. Cats are only species known to be affected (toxic to cats only).
• Asparagus Fern: allergic dermatitis with repeated dermal exposure. Berry ingestion could result in gastric upset (vomiting, abdominal pain, or diarrhea.)
• Begonia: Oral irritation, intense burning and irritation of mouth, tongue and lips, excessive drooling, vomiting, difficulty swallowing. Tubers are the most toxic.
• Baby's Breath: Vomiting, diarrhea
• Calla Lily: Oral irritation, intense burning and irritation of mouth, tongue and lips, excessive drooling, vomiting, difficulty swallowing.
• Corn Plant: Vomiting (occasionally with blood), depression, anorexia, hypersalivation, dilated pupils (cats).
• Cycads (Sago Palm, Fern Palm): Vomiting (may be bloody), dark stools, jaundice, increased thirst, bloody diarrhea, bruising, liver failure, death. 1-2 seeds can be fatal.
• Daffodil: Vomiting, salvation, diarrhea; large ingestions cause convulsions, low blood pressure, tremors and cardiac arrhythmias. Bulbs are the most poisonous part.
• Geranium: Vomiting, anorexia, depression, dermatitis.
• Jade Plant: Vomiting, depression, ataxia, slow heart rate (rare.)
• Pencil Cactus: Irritating to the mouth and stomach, sometimes causing vomiting, but generally over-rated in toxicity.
• Ribbon Plant (Corn Plant, Cornstalk Plant, Dracaena, Dragon Tree): Vomiting (occasionally with blood), depression, anorexia, hypersalivation, dilated pupils (cats).
• Tulip: Vomiting, depression, diarrhea, hypersalivation. Highest concentration of toxin in bulb.
The above is not meant to be a comprehensive list, but plants I've encountered pretty often without knowing their toxicity. For a full listing of toxic plants for pets, please see ASPCA's searchable database.