I have a few opto-isolator chips that were given to me (sorry, I can't remember what they are right now) which look like an opamp, but with six pins instead of eight.

Is there any guitar effects that use these kind of chips? What can I do with these chips?

I'd like to get some. I wsh Idda Tried a' order some samples with that TI order.
 I should try to find out the #[s?] of the chip[s?], look at the data sheet, and try to find where the deals are.
 Quite the interesting chip for use in musical effect/amplification IMO.
 What does it...1   Like a very nice' LDR?   2  Light/Variable Amplitude OA
 3 something else entirely

optoisolators R "usually" digital.
i.e.  "on/off".

there R optos that use jfet(s).
they R more expensive.
but, these make a variable resistance, suitable
for phasers, compressors, and vcas.

Mouser has both types in stock.
i think GEO has an article bout them...(?)
tone

Some such optoisolators can isolate and transform audio or dc level too, not just logig level or switching, but what I believe it is easy to put audio quality down with these. Poor signal to noise ratio, distortion and complex circuits maybe follows. I have only *very* little experience about topic, but I can tell I started but not finished a project for audio signal opto isolator. I worked in an emergency center and wanted to stay away from transformers because lots of computers, phone systems and digital and analog radio systems and cordless headsets. My circuit (used some "standard" optoisolator)  worked but there was bad level mismatch and had no time and interest to finish the project.

If you search for IL300 datasheet there is application for analog opto isolator. Maybe you could isolate your preamp totally from an unreliable and not always safe tube amp, or for stage use. If I had an old genuine JMI Vox from 60´s I perhaps would be happy to have galvanic isolation from it.