My current project consists of an Arduino with ethernet shield. Data is collected on the Arduino using RF sensors and analog sensors. These data are then transfered to a MySQL database on a server outside my house using the ethernet shield. On this server I can then view the data on a web page.
I was thinking of maybe saving the data locally on a SD card and represent it on a local webserver like for example on an Raspberry Pi.
Maybe I could connect a Raspberry Pi directly to the existing Arduino using the USB connection, and then install a MySQL database and a webserver on the Raspberry Pi.
In this way I could use the Arduino as an I/O extension for the Raspberry Pi.