Here are some Hobby Scopes that might even be quite professional if you develop them further. They are virtual instruments. Making Scopes with tablets may be much more easy now.
The µSCOPE – A poorman’s oscilloscope
The oscilloscope is still one of the most important measurement tools of the electronic engineer. With the advent of the often very reasonably priced USB scopes, such an instrument is now within reach of every body.
Digital oscilloscope – Build a simple Inexpensive digital oscilloscope. Single channel, about 100 MSPS. RS-232 based.
xoscope is a digital oscilloscope using input from a sound card orEsounD and/or a ProbeScope/osziFOX and will soon support Bitscopehardware. Includes 8 signal displays, variable time scale, math,memory, measurements, and file save/load.
xoscope – digital oscilloscope for Linux
MHZ100Q is a low cost FPGA-based high speed data acquisition system. Version 2 is a standalone board containing 2, 100MHz 8-bit A/D channels, preamp, 7th order elliptical antialiasing filters, and a Xilinx FPGA.
MHZ100Q – Open Source FPGA-based 100MHz A/D
A USB interface is implemented in the FPGA firmware. Power comes from the USB bus. Schematics, VHDL and Verilog source code, and signal capture and display software are available and being posted.