What are some good representative transistors for a beginner's electronics class?
Most of the following are fairly-generic common transistors that have been around for a reasonably long time and likely to be around for quite a while longer.  Most are available from popular distributors and cheap from Asia.



BJT Complementary Pair (NPN/PNP):


SS8050DBU / SS8550DBU : TO92, Ic = 1.5A, Vceo = 25V, "high" current for TO92.
2N3904 / 2N3906 : TO92, Ic = 0.2A, Vceo = 40V, cheap for low current needs.
2N4401 / 2N4403 : TO92, Ic = 0.6A, Vceo = 40V, some brands with Ic near SS8050DBU.
2N5401 / 2N5551 : TO92, Ic = 0.6A, Vceo = 150V, "higher" voltage.
MPSA42 / MPSA92 : TO92, Ic = 0.5A, Vceo = 300V, "high" voltage.
TIP2955/ TIP3055 : TO247, Ic = 15A, Vceo = 60V, common in older audio power amplifiers.




MOSFET:


2N7000 or 2N7002 : TO92 / SOT23, 0.2A, 60V, cheap for low current needs.
BS170 : TO92, 0.5A, 60V, cheap-ish but supports more current than 2N7000.
BSS138 : SOT23, 0.2A, 50V, Vgs(th) of 1.3V, common for cheap voltage-level translation, especially I2C.
IRFB7546PbF : TO220, 75A, 60V, 6mOhm, $0.53 at Q10 from Mouser on 2014sep01, ability to switch lots of current for low price.