LPC4370 cheap scope: 80Ms/s 12 bit LPC LINK2? £14 from Mouser.

[DAIRVINE]

80MHz 12 bit ADC processor LPC4370.
LPCxpresso do a LPC LINK2 and LABTOOLS open source oscilloscope demo set. (£84 from Mouser, seems quite well liked.)

The LPC LINK2 is £14 + VAT from Mouser, it has the ADC on board. The Lab tools board has signal conditioning, BNC, protection etc.
Using just the LPCLINK2 board:

Remove C14. Add a 27K resistor between the old C14 input pin and pin 2 of J4 (VDDA +3.3V). Replace R3 with a range set resistor, for example 100k for about +-10V input. (gives 5mV resolution.) The open circuit input will float up slightly. The input impedance of the ADC is specified as 5k in parallel with 4.5pF. Bandwidth will be around 10MHz.
Connect input to J4 pin 6 and a GND to pin 1.
There are a number of possible variants which may perform better or be easier to do or provide additional channels.
 

The open source software would have to be hacked as various peripherals on the lab tools board are missing.
I have not tried this proposed mod.

[DAIRVINE]

Review
http://www.drdobbs.com/embedded-systems/embedded-tools/240168337

Datasheet
http://www.nxp.com/documents/data_sheet/LPC4370.pdf
http://www.lpcware.com/lpclink2
http://www.embeddedartists.com/products/app/labtool.php

Source firmware and app:
https://github.com/embeddedartists/labtool

Relevant links
https://groups.google.com/forum/#!topic/labtool/Rtp3ZCPygfU