The two largest vendors of educational probeware are Vernier and Pasco:
Not every school has the ability/money to do probeware based labs, however. So, here is a description of how to create a really simple probe that will allow you to see the fluctuations in light caused by alternating current or see the signal from a TV remote control (as well as do other things I'll describe in a future Instructable).
Total cost is less than $5, but you need a computer with an oscilloscope program installed.
Visual Analyser - Free oscilloscope program for Windows. http://www.sillanumsoft.com/
Audacity - Can be used as a recording oscilloscope. http://audacity.sourceforge.net/
Step 1: What Do You Need?
-Cheap Solar Cell (I bought mine for $3)
-Headphones from the Dollar Store (you could just buy a microphone plug and some wire)
-Alligator Leads (I used red and black, but you can use any color)
-Electrical Tape (heat shrink tubing would be cooler, but I'm lazy and cheap)
Step 2: Destroy the Headphones
The colored strand carries the signal while the other strands are the common ground. You'll need to strip off the insulation. There are two ways of doing this. You can lightly sand the wire, but you risk breaking it, or you can burn it off. The enamel will burn off readily, but if you burn it too long the wire will melt.
Step 3: Destroy the Aligator Leads
Step 4: Put It Together
Be very careful not to break the signal wire. It's very thin and fairly easy to snap.
Solder it together.
Step 5: Hide Your Mistakes With Electrical Tape
Step 6: Using It.
You could solder the wires directly to the solar cell, but I typically don't. This way I can use my probe with any varying voltage source. Just be careful to keep the voltage low so you don't fry your sound card.
I've done some digging (after some comments, thanks) and found a reference to a 2Vpp voltage limit on a SoundBlaster AWE64 Audio Card. You should probably check your solar cell to determine it's maximum output. If you're concerned it would be a simple matter to include a current limiting resistor. The solar cells I've used are rated for a maximum of 0.5 V and I've had no problems so far.
-Any light plugged into alternating current
-Your computer monitor (frequency will equal refresh rate)
-TV remote control. The solar cell should pick up the IR.