The most common is Hypericum Perforatum (St. John's Wort). It's an (considerably strong) SSRI inhibitor and potent CYP450 inducer. Better not take it if you're regularly taking any other kind of drug. If you do ask your pharmacist/doctor, if you're already into any kind of anti-depressants also it's better to avoid it, excessive SSRI activity could lead to serotonin-syndrome.