regarding Freeware Effects : Once I really started looking into all those JS plugins I realised I could de-install 80 % of all the freeware-effects I collected. The JS Effects might not look nice but they get the job done fairly well. Never wanna mis stuff like JS/sweepinglowpass or JS/resonantlowpass any more, so cool stuff.
regarding Freeware Synths :
my favourite ones are iblit/polyiblit and Voyager (Oatmeal you already have)
Also search for soundbanks, especially for voyager there are many out there. if you don't find them, pm me and I send you.
http://www.kvraudio.com/get/366.html
http://www.kvraudio.com/get/577.html
http://www.kvraudio.com/get/1768.html
Oh and one effect I don't wanna miss ist TAL DUB II , my go to effect for delay :
http://www.kvraudio.com/get/3353.html
Check out the other TAL stuff, I like their Filter as well.
And dblue Glitch is a must if you need something totally weird effect-wise. Many producers use it, yet it's still freeware.
Another great Organ is ORGANized Trio :
http://www.kvraudio.com/get/682.html
I believe one could easily do full productions just with Reaper and freeware, yet I find a lot of payware is very comfortable to use and comes with tons of presets. That's why I buy payware as well.