Raspberry Pi is a fully functional computer, a System-on-Chip (SOC). With roughly the same price range as Arduino boards, it offers much more bang for the buck so to speak.
- Audio Injector (link), sound input/output for RPi.
- NanoPi (link)
- NanoPi is a device with 512MB RAM available for €22, or with 1GB RAM for €29.
- Analogue Sensors On The Raspberry Pi Using An MCP3008 (link)