The Steinberg VB-1 is a small and easy to use virtual bass plugin and is a very simple plugin with some surprisingly useful sounds. 

The Fullbucket MPS is a surprisingly easy to work with synthesizer, it aims to create sounds fast and easy using a small intuitive set of parameters.

A grand piano, Rhodes, Wurlitzer and Vibraphone in one free plugin.  With excelent effects and controls. A must have in your free collection.