For 1.7.10, Reika's mods would probably classify as lesser known, at least for people that joined the community more recently.
Otherwise, for 1.12, I think Nature's Aura is underappreciated, it's a magic mod based around naturally spawning Aura and ways to manipulate it. Better with Mods is pretty cool as well if you manage to configure it well (which is also part of the reason you don't see it that much). It's kind of an earlygame overhaul mod. A small but neat mod I recently found is Spotlights, which adds highly configurable beacon beams.
There is also Tetra, which is a not particularly often used TiCon alternative.
GamerwithnoGame's SGCraft reminds me of AUNIS, which also adds stargates. The models look a bit nicer, but it has a few missing features.
Finally, though the mod itself is not exactly unknown, Nuclearcraft: Overhauled is very cool and one of the best power gen mods out there. The overhauled version specifically makes building a reactor a complex but rewarding thing to do.