It seems that less money you have, more creative you are. Not many dear to start a DIY oscilloscope project since these tools already aren’t that expensive as they were earlier. Anyway there are always hobbyists somewhere in the world that would rather build a tool rather saving to buy. In my opinion, oscilloscope is must have tool on a bench and if possible good one. So if your budget is short to buy one, ajoyraman offers his plans on $15 scope.
Oscilloscope doesn’t come with display, so it has to be connected to PC. He made sure that software is a multi-platform where Visual basic was used to to build windows compatible part, while Python for Linux. Scope has two channels with sampling frequency from 1Mbps to 20Mbps. Other features are like on common scopes including triggering on both edges, selectable gains, different capturing modes including single, repeat and store. Most of work is done on PC software where you can select various parameters and see graphs. Here you can zoom, copy, save images as you would like. Scope is built around dsPIC30F2020 microcontroller that talks to PC through USB interface made of FT232R USB to serial converter. The front end of each channel is build by using MCP6S22 programmable gain amplifier controlled over SPI bus from microcontroller. The PCB is also home made and fits in to larger matchbox.