Transistors for switching and amplifying: MOSFETs (IRF3205, IRFZ44N, IRF540, 2N7000), general-purpose BJTs (BC547, BC557, 2N2222, 2N3904), Darlington, IGBT and power transistors such as the TIP41C. Check the voltage, current and package (TO-92, TO-220), and whether you need N-channel/NPN or P-channel/PNP.