Let's see, from the top of my head...
Red Power:
Solar Panel
Thermopile
Windmill (needs to be fairly high up, so you'll want a pair of RP voltage transformers and some high-voltage blue wire)
RP->BuildCraft: Blulectric Engine (can be converted to EU if you tack on the Power Converters mod)
BuildCraft:
Engines, from least to most power: Redstone, Stirling, Combustion; the last one has the chance of exploding if you don't feed it with enough water, 1 Thermal Expansion aqueous accumulator feeds 3 combustion engines adequately enough to prevent explosion, if I remember correctly.
Thermal Expansion:
Magmatic Engine (4 MJ/t, if it gets too hot, will shut down and require a whack with a BC-compatible wrench to restart)
Steam Engine(2 MJ/t, takes solid fuels like coal, has same overheating mechanic as magmatic engine)
Redstone Energy Cell (stores up to 600,000 MJ, adjustable input and output)
Forestry:
Electrical Engine (baseline of 6 EU/t in and 2 MJ/t out, upgradeable, accepts any voltage)
Peat-Fired Engine (1 MJ/t from regular peat, 2 MJ/t from bituminous peat, produces ash)
Biogas Engine (uses water, honey, milk, seed oil, and biomass [from least output to most]; produces 1-5 MJ/t, depending on fuel type, does not accept biofuel)
Railcraft:
Hobbyist's Steam Engine (1.2 MJ/t on solid fuels, 2 MJ/t when being fed steam thru waterproof pipes or liquiducts)
Commercial Steam Engine (4 MJ/t, must be supplied with steam from a boiler or tank via pipes/liquiducts)
Industrial Steam Engine (8 MJ/t, requires external steam source, same as the Commercial Steam Engine)
IndustrialCraft:
Generator (burns anything that works in a furnace, delivers 10 EU/t, burn duration depends on type of fuel used)
Wind Mill (highly variable energy output, very luck-based)
Water Mill (can either be fed water buckets or cells, or be immersed in water; buckets/cells produce more power than immersion, but require more logistics)
Solar Panel (1 EU/t on clear, sunny days; several varieties of superior output available depending on mod addons used)
Geothermal Generator (20 EU/t, require lava; can accept lava from BC pipes, TE liquiducts, or an adjacent BC pump)
Nuclear Reactor (variable, but incredibly high, EU output; highly dangerous, the wrong configuration can turn your base into a smoking crater)
Those are all that I can recall, and mechanics pulled from either the FTB wiki, or the given mod's own wiki.