I don't believe in the existence of parallel universes, but I do admit the concept makes for some fun stories. Out of the whole franchise, Stargate has only touched on this subject a few times. Note, I'm not including episodes that featured a time-travel created alternate timeline. SG's always depicted alternate timelines as something that overwrite reality rather than coexisting with.
1-19 "There But for the Grace of God"
3-6 "Point of View"
6-13 "Sight Unseen"
9-13 "Ripple Effect"
10-13 "The Road Not Taken"
"There But for the Grace of God" was great, because it was foreshadowing the season finale. We got a glimpse at what might very well happen. "Point of View" also another great episode, a sort of sequel to the other episode. It's a glimpse at what might have happened had key people not been killed and the SGC not self destructed. I don't know if you'd count "Sight Unseen," but people were seeing alien creatures from a parallel world, so might as well. I like how the Air Force swept this under the rug by passing the sightings as hallucinations. haha Next up: "Ripple Effect." I think we all loved this one. SG-1 returns to the SGC from a parallel world... again and again and again. haha Did anyone have trouble figuring out which SG-1 was the real SG-1? Last, we come to "The Road Not Taken." It was interesting to see what might have happened had the SGC gone public after the events of "Lost City." It seemed to imply that O'Neill was left in stasis at the outpost, Atlantis was never found, and Daniel and Vala still accidentally contracted the Ori.
Anyone notice that the last three "parallel universe" episodes were all the 13th episode of the season?
3-8 "McKay and Mrs. Miller"
5-4 "The Daedalus Variations"
I loved "McKay and Mrs. Miller." Two McKays, that was just fantastic. I often get frustrated with the writers favoring McKay, but the concept of this episode called for McKay to be the central character. I love how McKay became insecure about himself when Rod showed up. haha Notice McKay donned a leather jacket the following season? It seemed like he was trying to be a bit like Rod, no? haha I don't remember "The Daedalus Variations" well, I was surfing the web when it aired, and the plot didn't really keep my attention. I'll be seeing it on DVD soon enough, though. Finally, "Vegas." I didn't care for this. What was the point?
Do you think the show would have put out a parallel universe episode had it stuck around?