Scan are doing the FSP500-60HCN PSU for £19.80 atm, which for a 500w PSU from a known manufacturer seems a bit of a deal... so I bought one without looking too much into it.
It's not in front of me right now and I can't remember the specifics of the 3.3 and 5v rails, but it has two 12v rails each rated at 18A. It also says that 3.3v and 5v rails should not exceed 123w, leaving 377w (or 31.42a) for the 12v rails combined.

I don't know a great deal about PSUs, but I popped the top off and had a quick look - all the caps are Teapo, the primary cap is only rated for 85 degrees, but that's not unusual in cheaper PSUs.

All looking good so far to my untrained eye - a respectable amount of 12v power and reasonable quality components.

However the cables... Oh god the cables. They're short, unsleeved and fairly thin (not the thinnest I've seen though!), but the main problem is the connectors, or rather the lack of them. 24 pin mobo, 2x molex, 2x sata, 1x floppy and a 4 pin CPU connector. That's it. What on earth is the point of (what appears to be) a capable 500w PSU if it hasn't got the cables to power anything that would require a 500w PSU?! No pci-e, no 4+4pin CPU connector and such a paltry selection of other plugs is ridiculous on a 500w PSU!!

So yeah, don't buy this PSU unless you don't require 4+4 pin CPU power, PCI-E power and aren't running lots of drives or other accessories, in which case you'll probably be better off with a 300w PSU. A truly odd PSU and I don't know what FSP were thinking, but thankfully I've bent it to my will with molex splitters, molex->sata adaptors and molex->pci-e adaptors. I was somewhat dubious about switching it on with such a messy frankensetup, but it hasn't gone bang yet, in fact it's very quiet!