Does anyone know of some cartridges that use the /IRQ pin?
What is it typically used for?
I'd bet the same thing as on the NES: to let the cartridge generate an interrupt. It's less needed for raster effects than on the NES because of the built-in scanline counter and HDMA, but it's still there when needed. I seem to remember that the SA-1 can interrupt the main CPU and vice versa.
The SuperFX and SA-1 use it to trigger CPU-side IRQs when they complete certain processing events (eg SuperFX executes STOP, SA-1 writes to a special register, trips its internal time counter, or completes a type of DMA.) Obviously, there's no /NMI pin so you can't trip NMIs on the CPU-side. The CPU can distinguish CPU-native IRQs from special-chip-generated IRQs by checking the special-chip MMIO IRQ-acknowledge registers. What's not entirely clear is how long the pin is held for, I believe it stays held until said acknowledge registers are read.