Many free virtual instruments and effects are promotional versions of premium plugins and are made to the same level of quality. They may have a reduced feature set or some limited options but they are going to be great for your music-making.
Some others are developed by enthusiasts who don’t have the backing of a brand and they can look a bit rough around the edges but also they can be full of creativity and original ideas. Besides, any plugin listed here is going to be top-notch.