Free plugins, VSTs, samples, loops, and more. These are all good quality and reputable companies.
Many great plugins and samples are released every month. They may be trial versions for limited periods or with reduced functionality, or full-blown software.
In any case, you can try before you buy, or just get the goods for free. Enjoy!
VSTs (Virtual Instruments)
Put an orchestra or scorching synth in your computer for free.
For those old enough to remember the Realistic MG-1 synthesizer from in Radio Shack stores in the 80s. Get it here.
Be sure to see our section on free samples and packs for Novation Circuit.