For my synth parts in FL, I rely exclusively on Superwave P8 and Synth1 (Also free -- A must-download for the low-budget FL user).
These two synths can both produce some mean leads once you know how to use them.
Superwave and Synth1 should, together, be all you need in the way of freebie synths, but you could supplement those, if you like, with Triangle II and FreeAlpha (Both also free). Pick one you like and work up your skills with it.
Evidence that these synths are capable of some big leads:
The Superwave is the easier of the two to use, and can produce some great bread-and-butter sounds, but nothing too interesting. Synth1 is harder to use, but can create some far out leads, basses, and FX, though sometimes it has aliasing problems that muddy up its output.
All that being said, it's pretty easy to make the classic JP-80x0 supersaw sound with Superwave. Here's how:
-Create a new Superwave channel
-Set both oscillators to be sawtooth waves.
-Change both oscillators modes to Superwave
-Push each filter's resonance about 2/3 the way up.
-Pan one oscillator all the way to the left, and the other all the way to the right.
Bam! Instant big saw lead.
-Create a new Synth1 channel
-Set the attack/release on each envelope to zero. Set the sustains to max.
-Set the filter cutoff frequency to its maximum.
-Raise the flanger depth to 96
-Toggle Mono mode.
-Toggle Unison mode.
Bam again!
On another note, if you're in the market for free effects to complement those provided by FL, you might also want to check out:
Smartelectronix's Ambience
Voxengo's
TubeAmp
Elemental Audio's Inspector
Kjaerhus Audio's effects packs
MDA's effects packs
Good luck, and enjoy!
