Most of the mixed photographic solutions are not very dangerous, and neither are most of the unmixed packaged developers and fixers. That doesn't mean you wouldn't get sick from them, but they are not lethal acute poisons. Most manufacturers do an excellent job of telling you about any hazards on the label (at least in the US). The most hazardous are probably the toners, some of which are acutely poisonous. That does not apply to the raw components you might use to mix up your own solutions, however. Here the chemicals are much more concentrated and can be more toxic. Also, the bottles of raw chemicals are not usually marked to indicate poisons. Hydroxides have already been mentioned as nasty. Add glacial acetic acid to that. Developing agents like pyro and PPD are quite toxic and can be absorbed through the skin. Sodium sulfide (the toning agent, not sulfite) is quite toxic. There are a number of other things that show up in an experimenter's darkroom that should not be ingested, but these are the most common.