If you want quick and dirty just use something like the ADS7830, it runs on i2c so if you want minimal soldering just run it direct from a i2c bridge or even a bus pirate and have the monitoring app as a daemon on the pc. 8bit so 0.05v step resolution if you were wanting to monitor 0-12v.
You'll need to set up some lot tolerance potential dividers to cope with the 12v and you may run into problems with shared grounds depending on what you are monitoring. I *think* the 7830 has an isolated analogue signal ground so you may just end up needing a couple of them running on different ground planes but you'll need to check that on the datasheets. Otherwise it probably supports 4 channels of differential measurement so you could do the ground isolation that way.
Alternatively pick any microcontroller platform and build a standalone monitor that buffers and spits out the data via serial or whatever. Doesn't matter what really, could be PIC, AVR, or even just an Arduino shield, the right Arduino shield/AVR has 8 ADC inputs on chip but then you have more work to do on ground isolation probably.
Either way a fairly trivial amount of code and a tiny bit of hardware would see you though. Would take a bit more effort to get it spot on and stable and again depending on what you are monitoring you may want some electrical isolation between your PC and whatever is being monitored.