If I said to you that a transistor can be used as a couple of diodes you would think that is kind of boring. You'd be surprized that there are a lot of things you can do using a transistor as a 2 terminal device:
One thing I didn't mention is (if I'm not wrong) you can use 2 transistors to make a full wave bridge rectifier. That would only be a novelty item because you would have to keep the input voltage below 5V.
Thanks, lots of useful info. Just need to remember the voltage limitations of the various junctions. Rich