Linear Technology LTspice:
A free op-amp simulation tool
Mancini, R., ed. Op Amps for Everyone: Design Reference. Texas Instruments, 2002. (PDF - 2.0MB)
An excellent reference book for op-amp design.
OpAmps: Active Filter Synthesis Design Tool
A simple tool from Analog Devices for designing active filters using voltage-feedback op-amps.
Sensors Web Portal
References, hints, sources from the International Frequency Sensor Association (IFSA)
Robotics related products compiled by Kevin Dowling
Bill Buxton's Directory of Sources for Input Technologies
Big trade show with tutorials and proceedings
Very large new state-of-the-art sensors conference
Old standby conference for sensors and applications
Emphasizes MEMs, but like IEEE Sensors
ACM conference on user interface technology