FL Studio Producer edition or higher includes an FL native version of SynthMaker. SynthMaker allows you to create your own instrument, effect and dashboard plugins. There's also a growing collection of user-made freebies, see the bottom of this news item.
You can find SynthMaker under the FL Studio menu Channels > Add one > SynthMaker
If you aren't familiar with SynthMaker then crack open the pop-corn and sit back for this series of tutorials showing how to roll your own instrument and effect plugins:
FL SynthMaker 2 Oscillator Synth
The video above has a 'follow on' tag at the end to Parts 2 and then 3
FL SynthMaker Effects
The video above has a 'follow on' tag at the end to Part 2
However you don't need to be a synthesis guru to use SynthMaker as you can download a bunch of great plugins from within SynthMaker. Just click on the 'LIBRARY' button and select from those available in the Downloader. Click the 'SHOW PANEL' to swap between the plugin interface and it's underpants.
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