ICubeX from Infusion Systems - the I-CubeX converts input from a wide variety of sensors (sliders, buttons, flex, pressure, distance and more) and to MIDI messages that can be used to control Isadora or XMidi.
MidiTron: Eric Singer's MidiTron is an inexpensive board that takes analog input and converts it to MIDI for use within Isadora. The manual contains instructions on how to connect various types of sensors to the MidiTron.
MIDISense: Limor Fried also creates inespensive analog to MIDI boards. See information about her open-source MIDISense project here.
Doepfer Electronics - Take a look at their "Pocket" series of MIDI sliders as well as the DIY (Do-It-Yourself) Pocket Electronic.
If yoiu are interested in creating your own sensory hardware (or even if you just need to know how to solder) take a look at the NYU's marvelous Physical Computing Page.