Robotics without a budget...Sensor suggestions?

Hello all!
I'm about two thirds through my first robotics project. The layout is like this (ALL of the hardware is completed at this point!)-
Frame - 2 Plexiglass sheets, with aluminum strips for the brackets and mounting points (It's so ugly I'm thinking about not taking any pictures of it...but it didn't cost anything, and it works!)
Motors - 1 GM3 and 1 GM2 from solarbotics (Same motors, but one is inline, the other is offset. I don't know why i didn't get 2 of the same.)
Motor driver - 2 6-transistor H-bridges (some transistors were salvaged)
Batteries - 4 1.2v 2000mAh AA NiMH batteries, arranged as 2 sets of 2, in series...1 to power the control circuits, the other the motors.
Brain - PIC16LF873A or PIC18LF272 (I haven't decided which one for final design. right now i'm using the 873A. I may use both, using serial communications over PORTC)
Sensors - 3 IR object detectors (salvaged from MS mouse wheel encoders, 2 in front, 1 in back), 2 photoresistors. These currently take up all 5 of the analog inputs I have available :(. I may put in 2 quadrature encoders on some of the digital ports, once I figure out how the hell to mount them on my motors.
The software is the final third of the project, which I don't anticipate any problems with. But i'd like to have some more sensors, and I've really run out of ideas. I have an absolutely crappy job, so I don't have any money to spend, at all (everything from this bot was either free, salvaged, or junk I have laying around). If anyone has any ideas on salvagable sensors...I'd really appreciate it!
-Alex
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What kind of sensors ? This depends very much on what you want to accomplish. I know some people have home-built magnetic compasses ( buoyant needle in liquid, coupled to wheel encoder ) For odometry, some people have used optical mice, there's a tiny little camera ( 16x16 pixels, IIRC ) underneath them. If you are short of inputs, you can multiplex some. Potentiometers rigged with some mechanical systems can work as position/force sensors. Heat sensors can be made of various thermoelements. I've seen projects with salvaged lasers from CD drives.
google for details
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People throw away printers and VCR's all the time- plenty of gizmos in there
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