This Google Chrome extension dynamically injects the Edit Area code editor into salesforce pages where formulas are edited.  Install the extension and when you visit a formula edit page it will automatically enhance the textarea.

Chrome Web Store Link

Features

• Syntax highlighting
• Tabbing (tab and shift-tab)
• Parenthesis matching
• Find and Replace
• No code wrapping
• Resize editor window by clicking bottom right corner
• Full screen feature

Downsides

• The “Insert Field” buttons will no longer insert the field. You can still browse the fields and then copy and paste the resulting field names though.