CodeMirror - In-browser code editing

CodeMirror is a JavaScript library that can be used to create a relatively pleasant editor interface for code-like content. It could be easily integrated in to your webpage. It supports JavaScript, XML/HTML, CSS, Python, PHP, diff, C, Java, sTeX, LaTeX, Haskell, Smalltalk, reStructuredText and PL/SQL. It also supports features like Autocompletion, Search/replace, Highlighting the current line.