Having done this before and STILL not knowing how to do all that youtube embedding, spoilery stuff, rehashing on a theme with plentiful text for your googling delight...
Obligatory:
M J Hibbett and the Validators. Second main album "This is not a Library" is
particularly choice, but debut (Say it With Words) and third (We Validate!) are also rock solid for practically the whole running time; I haven't got round to opening the packaging on the fourth (been busy

) and there's a whole pile of EPs and the like that I'll skip merely for brevity. The mixes are sometimes muddy, but understandable - the band leader is sort of a label owner, but I'm not sure if it counts (Artists Against Success basically being an intermediary for indie artists and disc pressing plants, and somewhere to have your recording index published so you can seem a touch more pro), and everyone is essentially a hobbyist. Their songs theme my life quite intensely.
Other obligatories who sort of count:
Spray and/or
Cuban boys and their component parts - electro/goth/sampleariffic noise pop lunatics of the infectious kind. Were signed by EMI ... and dropped after being neglected. May or may not be signed to Ninthwave records now. Still very much indie all the same, doing their own releases of most of their material and mostly turning up on 9thwave samplers as well-known roster-boosters rather than doing full albums with them.
You may know the former band from I Am Gothic (and its DDR-troubling tendrils) or maybe the Raccoons theme cover or "Everything's Better With Muppets", the latter from various Peel sessions, the theme to "Still Game" and, of course, "that bloody hamster song with the sweary radio announcer (no not the rubbish Hampton travesty) that managed a good showing against Cliff and Westlife for the millennium xmas single spot".
(If any of the above are watching, and I'm making serious mistakes, I can only apologise. Reminds me, I
was in conversation with Ric from Spray about getting hold of their more recent stuff til my email imploded, must get that sorted and see if he's given up on me)
And... hmm. My knowledge has fallen off a bit recently, into a haze of chiptunes, nerdcore and more commercial stuff. But I can highly recommend you check out
Dubmood and
505 (chiptune proponents of the YM2192 with occasional extra accompaniment, rather than NES stuff, so they get less love despite the nicer sound

plus an honorary mention for
gWem who made the handy soundtracker all the cool atari composers are using these days and is reasonably good with it himself),
Frontalot/MC Chris/Rappy Mcrapperson of course (the latter is... somewhat spoofy),
Lemon Demon, and upcoming UK act of some interest,
Milk Plus... who I'm not sure is entirely nerdcore, but I've played the debut album leak too many times and will happily buy it on final release. He can lay rhymes on some geeky and generally non gangsta subjects, but seems to be just at home with down-to-earth non geek stuff and more tuneful, slow paced flows. Apparently signed, but I've a feeling that much like Hibbett et al, the label is one that he partly has a hand in.
Also picked up at random at a beer festival an apparently self made (but to a very high standard) record by
Doctor Busker. Numerous traditional, largely folksie type tunes but given modern interpretations on the lyrical side from the perspective of someone involved in a lot of preservation rallies and the like, with a rather dirty mind and mouth. I think the young people on the streets would probably call it Steamcore. Good luck finding it on the internet, I haven't even bothered trying. You probably need to download it at 56wpm over telegraph or something.
If you're after entirely imaginary music, get down with my mate's proposed eurovision-entry band (includes me on keys/brass/whatever, which immediately dooms it)
The Love Love, with songs about the confectionary olympics and other matters of high import.