WriteLaTeX is an on-line collaborative LaTeX editor. It is written and maintained by John Lees-Miller and John Hammersley. It first went live on September 30, 2011, and it's been improving ever since.
Every document you create has a secret link. Just send it to your co-authors, and they can review, comment and edit.
Create Beautiful Documents Together
writeLaTeX synchronizes changes from all authors transparently, so everyone always has the latest version. Works great with the todonotes package!
writeLaTeX compiles your document in the background, so you can see the results right away. It's great for learning — quickly tame even the most complicated equations!
Not Just Papers
Make amazing presentations with the beamer package! The preview also helps with complicated tables, tikz figures and pgfplots graphs.
Find Errors Fast
writeLaTeX shows you errors and warnings as you go, so you can catch them early, and it shows them inline, so you don't have to find them in LaTeX log.
LaTeX in the Cloud
Edit online from anywhere and on any device. There's no software to install. Easily upload figures, bibliographies, classes and styles.