To clarify, you don't usually see just "a" for an accumulator, as there are usually more than one accumulator-style registers in a CPU, and in many cases they are split along byte (possibly word) boundaries.
So you end up with accumulators called "A" and "B", but are composed of registers "AX" and "AY", and "BX" and "BY", with each being one byte (or word) wide; X and Y being high and low bytes/words of the register (and dependent on "endian-ess" too).
Sometimes you even get where multiple registers can be referenced by a singular name - "D" is a popular choice, and may be made up of "A" and "B" (being low/high "registers" of the larger word). IIRC, the 6809 was like this (?) - A and B were 16 bit registers, but could be referenced as a 32-bit word "D" (or maybe I am thing of the 68k or some other architecture - it's been a long while).
The only other time I have ever seen singular letters used for registers in assembly was for very old pre-microcomputer systems (beasts like the Univac and System/360 - though I think the PDP-8 had similar style). Also some of the very early "microcontrollers" (which were more like glorified sequencers with some extra memory and rudimentary branching, if any) had similar "registers" (Radio Shack once sold, as a part of their "Science Fair" electronic kits, a "Microcomputer Trainer" that was something like a very small 4-bit microcontroller with 128 bytes of memory or something like that - to teach assembler and a bit of hardware interfacing - it had "small" registers like that referred to in single letters).
The 8080 had A, B, C, D, E, H and L. These mostly carried over to the 8085. Newer chips have ax/al/ah, eax, rax type names the grew out of the original names. The Zilog Z80 and Sharp LR35902 were mostly 8080 compatible.
The MOS 6502 has, as gmfawcett said, single-letter names. These in turn carried over to Western Design Center (WDC)'s 65C816. There are actually separate instructions for loading and storing in A, X, Y and Z at least on the '816. LDX, STX, and so on. This means the Ricoh 2A03, Ricoh 5A22, Hitachi 6309, MOS 8501, MOS 8502, and the later MOS 65xx series and the CSG chips. A fun fact is that the 6502 had especially fast access to its zero page memory and special instructions for some functions on that page, the first 256 bytes of RAM. Language implementers sometimes made up for the dearth of registers by treating certain addresses in the zero page as additional registers.
The Motorola 6800 had two accumulators, A and B. The stack pointer was merely S. X is the index register. It also treats the zero page specially. The 68000 series broke with this, having eight address registers a0-a7 and eight data registers d0-d7.
All of the above used A as an accumulator at least by convention in the materials.
SP is the literal name of the stack pointer on x86 in 16-bit mode. It's also used as an alias for R13 in at least some Arm (AArch32 on v7 and earlier for example). SP and PC are the stack pointer and program counter on the PDP-11. It's aliased to r1 on the Intel 80960 (i960) since that is the stack pointer on that platform.
The PDP-8 used similar zero-page tricks to the MOS 6502, only given that it had one (1 !!!) register, that was necessary.
All of these processors where CPUs for commercially successful systems. They might "only" be microcontrollers today.
The MOS 6502 / 6510 and its variant the WDC 65C816 was in the Commodore 64, Commodore PET, the Vic-20, the Apple II, the Atari 2600, the Atari 400/800/600XL/800XL/1200XL/800XE/65XE/130XE, Nintendo Famicom, SuperFamicom, the NES, the SuperNES, BBC Micro, Ohio Scientific Challenger 4, Atari Lynx, Apple III, Apple IIgs, Acorn Atom, Acorn Electron, Franklin Ace, and loads of clones.
The Z80 was in most Amstrad models, in the original TRS-80, the MSX standard, VTech Laser, Intercompex Hobbit, Mattel Aquarius, the Microbee, the NEC PC-6000 & PC-8800 series, Sinclair ZX line & Timex Sinclair, Coleco Adam, and again a bunch of clones.
The Motorola 6809 was in the Tandy Color Computer, while the smaller CoCo MC-10 used the 6803. A few other companies built around this chip family, too.
The Commodore 128 featured both a 6500 series processor and a Z80.
Several of these processors still have versions produced in 2020, although they're not for your main desktop or your phone. Several of them are targets for emulation or new hobbyist software due to the popularity of their platforms. And yes, some of them are used as microcontrollers. Microcontrollers need code written for them, too.
