With Cyrille, we added 8 analog input to our synth on RPi for only 13€ thanks to the Byron !
More info here : http://www.1010.co.uk/org/byron.html

Here is how to get started with Byron on RPi.
Prepare the system :
sudo apt-get install libusb-dev avrdude avr-libc

Change the permissions settings to allow access to byron to normal user :
sudo leafpad /etc/udev/rules.d/41-byron.rules
add this :
ATTRS{idVendor}=="16c0",ATTRS{idProduct}=="27d8",MODE="6666",GROUP="pi"
save
unplug / replug the byron

Download byron (the address in the first line might be tuned if a newer version is available, see the link above) :
wget http://1010.co.uk/byron005.tar.gz
uncompress the file, change to the directory and make :
tar -xzf byron005.tar.gz
cd byron/bootloader/byronjump
make clean
make
cd ../../programmer/firmware
make clean
make main.hex
cd ../../bootloader
./byron.sh
choose byron mode

cd ../puredata
rm byron.pd_linux
leafpad byron.c

change in line 129 10 to 19
change in line 11 40 to 20 (to speed things up)
make
pd byron.help.pd

That’s all !
Have fun !