MultiMarkdown Composer won't launch?
First, all known crashes have been fixed. It's been months since I've received a crash report. When Composer just refuses to launch, it is almost always due to one of two things:
Missing receipt from the Mac App Store.
Corrupted preference file.
When you purchase an application from the Mac App Store, it comes with an embedded "receipt" file that is keyed to your specific computer. It is not valid on other computers, so it can't be copied from one to another. In order to run MultiMarkdown Composer, you have to download a copy from the Mac App Store.
The "non-sandboxed" build can be downloaded from the web site, and uses the same license key as the MAS version. If you installed the MAS version, the alternate build looks for the receipt there and keeps a copy. If the MAS version is installed somewhere unusual, you can run it at least once to store a copy of the receipt in a standard location for the alternate build to search.
These receipts are copied and stored in two places:
/Users/<user>/Library/Application Support/MultiMarkdown Composer/receipt
/Users/<user>/Library/Containers/com.multimarkdown.composer.mac/Data/Library/Application Support/MultiMarkdown Composer/receipt
If MultiMarkdown Composer can't find a valid receipt, it exits and you can see the error code 173 if you look in the Console application and search for "MultiMarkdown Composer".
If this happens, you can delete the invalid receipt files from
the above locations and ensure that you have a valid version of
Composer from the Mac App Store installed in
/Applications. The next time you launch either
version, the new receipt will be located and copied.
NOTE: In running various versions of Composer for several years, on an almost daily basis, and doing all sorts of crazy development stuff, I have never had a receipt "disappear" or "get corrupted." When I get a support request from someone saying that "Composer just stopped working", after some digging it has almost always been because of something like copying an app to a new computer, deleting all the preferences and receipt files without having a MAS build installed, etc.
The other thing that can happen is that preference files can be corrupted. In this case, you should delete both the MAS prefs and the non-sandboxed prefs. You should also immediately reboot your machine. This sounds strange, but there is apparently a bug in OS X 10.9 (maybe 10.8??) that results in preferences not actually disappearing when you delete the files. See the following page for more information:
Lastly, and much less common, it is possible that you tried to open a problematic file. This caused Composer to crash (again, I have not heard of this happening for a long time -- let me know if you find a file like this). Because of the Auto Save/Restore settings, when Composer relaunches, it tries to open the difficult file, leading to another crash. Again, in this circumstance the Console application is your friend to identify the source of the problem.