Everyone who uses Mekinism knows about the Hydrogen Generation system which is fueled by water and can power a base of decent size, assuming you can kickstart the setup anytime you have no power. But Engineer's Tool Box has an addon called Gascraft with a way to generate Hydrogen for free, called the Photobioreactor. It needs Algae, a resource that can be easily automated in ETB for free, or just gather in massive amounts by finding a swamp and grabbing it all. You also need water, aka easy to obtain. The whole thing costs very little to get started, requiring an ender pearl as the most expensive item, and even GasCraft has a way to power your base with just hydrogen, with it's two gas-powered generator sockets. One is kinda meh, generating the same amount of power for all gases for a somewhat short time that doesn't change per gas, and one that has a variable output but only works on some gasses. The second one even can generate smoke for you if you set things up correctly.
Also throwing my hat in for Golems for TC4.1, way easier to get into, and still just as cool.
Also worth trying out is Blood Magic and it's Lava Crystal. Most people go straight for lava powering systems and tell you all about the Serenade of the Nether, and it's pretty sweet, I won't lie. But earlier then that is a T1 item called the Lava Crystal, which consumes 25 LP for one "smelt" in a furnace, aka the amount of power needed to smelt one item in a furnace. Coal is worth 8 smelt, wood 1.5, etc etc. But while Lava is great, not every system will allow it's use. But the Lava Crystal works in anything that uses "generic furnace fuel", so powering anything with a bit of blood is easy if you can use normal furnace fuels to power it. Best of all? It won't kill you if you run out of LP, just make you nauseous. Hence why when I finally end up doing my bit in Refuge to Regent Challenge, I will allow for somewhat techy things, if I power then via arcane means.