Trap has simple sound needs that make it an especially suitable genre for freeware. There are a lot of choices, but here are a few possibilities I swear by for freeware trap production:
Vital - Freeware wavetable synth on par with commercial synths. Simple interface with lots of free trap presets available.
An alternative is Surge XT. Also free and actually more powerful but a bit more complicated.
Spitfire offers BBC Discovery for orchestral sounds. Small subset of their larger offerings but more than good enough for trap.
Spitfire also offers their free labs series which has a wide range of traditional and not so traditional instruments. A few quality piano and keys.
Ik multimedia - sample tank and syntronik free versions. The sounds are a bit thin compared to instruments like omnisphere, but they are fine for trap and you can always layer.
Kontakt player includes some decent stock instruments ranging from band to vintage that are very usable.
Sitala is simple but free. For trap drums you typically need only snare, clap, closed and open high hats and a few perc sounds.
Another option is MPC Beats. I find it more useful for boombap or griselda/alchemist type sample based beats but it really can be used for anything. It does have a higher learning curve but offers much more.
Effects- the Meldaproduction free bundle covers tons
In addition i recommend the tdr nova dynamic eq
The airwindows plugins are as no frills as they get but excellent
Freeclip by venn is also something useful for trap