Commodore VIC20 and 64
Both of these Commodore machines run basically the same system - the main difference being that the 64 has more memory and colours,better sound and sprites - animated objects that can be moved around the screen.
The VIC,whose name comes from Video Interface Chip,comes with 3K of onboard RAM - which is paltry by today's standards,but compared to the ZX81 during the heyday of 8bit computing - it had 3 times as much as the competition and colour and sound - which made it the better buy.
These days they are going for around £20 on Ebay and can still be viable computing machines - I have both models linked to synthesizers via interfaces I made courtesy of MAPLIN [see members.lycos.co.uk/templarseries/maplin.html].
Both models use the same keyboard which has 8 function keys accessed with the SHIFT key to obtain all 8 extra keys - ROM graphics are available,but RAM graphics is a matter of moving the character set to RAM and putting the characters on the screen - or use the Super Expander cartridge (VIC).
The VIC has 3 sound generators and a fourth for noise - all three generate square waves and any ADSR or vibrato is done under program control,there is one volume register controlling overall volume.
The screen is 22x23 which is rather thick and chunky.8 colours are the norm,though the screen itself can be one of 16.
The VIC has a rather reliable track record which is why they are now referred to as "trusty VICs" - the good thing is - you do not need to wait around for your Op system to load,which you do with Windows - some things have not improved with age - the Commodore has stood the test of time - which is why you can still buy working models on Ebay.
The sound is much improved on the C64 - a whole chip (SID) given over to sound generation - this has multiple waveforms (Pulse,Triangle,Saw) and a filter as well as ADSR wave shaping and modulation.
The screen is improved on the C64 with 16 colours being the standard - double the VICs - and the layout is 40x25 which is much more readable than the VIC and more viable for wordprocessing.The C64 supports sprites - which are animated objects - this is all done under program control using registers which is not the easiest way to animate anything - but it is better than the VIC.
Note that complete memory maps and machine code guides are not in the original manuals - but they can be downloaded from the net - look for "6502 processor commands" and "C64 memory map" or "VIC 20 memory map".
Both machines are excellent and easy to program for hobby electronics enthusiasts and I have been using them for decades to control circuits using their user ports - see the above page on MIDI control to find out how to use them with modern MIDI synthesizers.
Things to look for on Ebay:
Watch out for a couple of problems when buying from Ebay.
Some models have differing cassette connectors - European models may have a small DIN plug - others use a flat connector - make sure the cassette and main unit have the same plug and socket. [The C16 and PLUS4 maybe subject to this too]
There has been variation in the power supply plug and socket - make sure these match.
Dodgy modulators may render the TV picture illegible - you can get replacements - but check for the modulator status. The C64 has one built in and whilst I have connected the VIC20 to the CV socket of a TV and bypassed the modulator altogether,the C64 video outlet is not compatible and so an aerial inlet and tuned channel is needed.Either that or a viable monitor (see the C64 user guide),or an S-video cable, which you can find on Ebay.
Some models can suffer from overused keys which have become sticky or unuseable - it is quite easy to combine the keyboard from one model with the CPU of another to get around this problem.Check for dodgy or sticky keys.
Both models have cartridge slots - in the VIC's case this is used for memory upgrade and without a 1010 motherboard you can only have one cartridge at a time,the most useful of which is the "switchable RAM" - these can be found on Ebay. Be careful that the model you buy has not got a worn cartridge socket.
Both machines have game ports of the "D" variety - the C64 has two - the VIC only one - these are incompatible with ATARI joysticks so do not use one with the other.