I know that there are poisonous amphibians such as the poison dart frog. However, are there any venomous amphibians .i.e. that inject a toxin via a bite or other method?

A number of salamandrid salamanders (most notably the sharp-ribbed newts and crocodile newts) have poison glands on their flanks positioned adjacent to their rib tips. When picked up by a predator, the newt can extend the rib tip outwards through the poison gland, thereby using the sharp rib tip as a poisonous spike. So far as we know however, no amphibian has a venomous bite.