Telefunken TR4 (Algol-60, FORTRAN-IV)
IBM 370, CDC 6600 (Fortran)
Sinclair ZX-80 - my first private machine with 1-16kB RAM,integer/FP BASIC; ROM firmware studied with disassembler programmed in BASIC, hardware by tracing board layout (model 80 had standard TTL ICs). Good exercise for learning to modify hardware and for developing machine code routines used in early "El Bicho" control; where timing loops only needed correction for the difference in CPU clock frequency.
Tandy II (BASIC, Z80 machine code)
Apple][, PC-AT (BASIC, Assembler)
PC hardware
In the more recent past, growing interest in PC networking under Linux (initially, Ubuntu/Debian, kernel 2.6.10, ISA net cards and sound hardware requiring manual setup..., and all the other stuff one learns operating one's home computer network, and repairing hardware and, occasionally, "Micro-software").