The only things you can't move from the C drive are the core Windows files. Things like your Steam library (or any other games), movies, pictures, documents, downloads, and almost any installed program can be moved. Keep in mind that any program you move with a shortcut will need to have the shortcut location changed.
I highly recommend a small program called WinDirStat to help you determine what is taking up the most space. It constructs a visual representation of the contents of your hard drive, very handy for situations like yours.