The best free alternative to Overleaf is LyX, which is also Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Overleaf and six of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Overleaf are TeXstudio, Curvenote, TexitEasy and JLatexEditor.
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.
TeXstudio is an integrated environment for writing LaTeX documents. Our goal is to make writing LaTeX as easy and comfortable as possible. Therefore TeXstudio has numerous features like syntax-highlighting, integrated viewer, reference checking and various assistants.
Curvenote is a collaborative writing tool designed for open-science and reproducibility. Create & edit technical content in real-time, including equations, references, figures, and interactive plots from Jupyter, then export to any journal template.
TexitEasy is a free, cross-platform and open-source latex editor. TexitEasy has all the features a modern editor can have.
JLatexEditor is a cross-platform open source LaTeX editor. It creates a unique development environment for a user by integrating many tools needed to write LaTeX documents, and is highly configurable.
TeXPortal is an application to typeset TeX documents, targeting mainly at academic researchers.
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Combine lyx with git to enable collaboration.