What open source softwares are available for creating a music score? I have Logic X but I don't think it's geared well for that kind of thing.
Since you're looking for software to input a score that is still under construction, MuseScore (found at musescore.org) would be my go-to application. It's a GNU-licensed graphical score editor that has playback and range-checking abilities.
In case you later want to engrave a finished score with LaTeX-like typographic quality, LilyPond is considered to be one of the finest software packages you can use. Do note that along with LaTeX quality comes a LaTeX-like workflow of entering the music into text files and then compiling them into the PDF output. However, is uses different syntax than LaTeX due to differences between the needs of typesetting text and engraving music.
Another good option which I've used (before buying Sibelius) is Finale Notepad. There are paid and free versions, but the free version (IIRC) doesn't have that many limits, and I could compose quite happily with the free version.
Note that this doesn't satisfy the original question's requirement for Open Source software, but could be an option for people if they don't have the same restriction.
If you are looking for browser based open-source software, I am currently working on inknote. It is currently incomplete, but if you are looking for something you can easily contribute to, this is it :)