The Hex Editor is a very powerful memory viewing/editing tool, it obsoletes the Memory Viewer tool from the FCE Ultra and FCEU Rerecording branches.
It can do a wide range of things. It allows you to view the entire RAM & ROM contents in a resizable dialog window. It makes it easy to edit the game’s RAM, PPU memory, and even its currently-loaded ROM data by simply typing in values in the editor. You can also “freeze” parts of RAM (to prevent the game from modifying the data there), search for data (Ctrl+F), and even copy and paste data to/from the clipboard. Furthermore, table files are supported, so you can edit a game’s text in real-time and see the result immediately.
Basically, it lets you tinker with any part of a game’s RAM or ROM while it is running.