Cassiopeia is described as 'Scientific Wordprocessing Software for MacOS X, MathML based equation editor, LaTeX' and is a LaTeX Editor in the Education & Reference category. There are more than 10 alternatives to Cassiopeia for a variety of platforms, including Windows, Linux, Online / Web-based, Mac and Self-Hosted solutions. The best alternative is LyX, which is both free and Open Source. Other great apps like Cassiopeia are MathType (Paid), KLatexFormula (Free, Open Source), Gummi (Free, Open Source) and VerbTeX LaTeX Editor (Freemium).
LyX is a document processor that encourages an approach to writing based on the structure of your documents (WYSIWYM) and not simply their appearance (WYSIWYG). LyX combines the power and flexibility of TeX/LaTeX with the ease of use of a GUI.
MathType is a powerful interactive equation editor for Windows and Macintosh that lets you create mathematical notation for word processing, web pages, desktop publishing, presentations, elearning, and for TeX, LaTeX, and MathML documents.
Gummi is a lightweight LaTeX editor for the Linux/GTK+ platform, released as free open-source software. It is designed with simplicity in mind and includes features that will make a novice LaTeX user quickly become accustomed to this mark-up language.
LaTeXiT is a small utility that allows you to quickly typeset LaTeX equations, without bothering with file creation, preambles, and so on. The PDF image obtained can then be exported by dragn drop to any application supporting it.
Tex2Img is a free online Latex equation editor that converts Latex equations to high resolution images to embed in documents and presentations. You can also use Tex2Img to learn Latex by trial and error.