Main feature:
Merge all custom dictionaries to keep them unified

Things to consider:
Formatting of the files (Make custom formats possible)
Have a list of common custom dictionary locations
List of words generally not in these dictionaries.
Same file format? Use filesystem junctions/hardlinks instead of duplicate files.

Sources to consider:
(a mobile app with similar function)
https://play.google.com/store/apps/details?id=com.usr.dict.mgr&hl=en
(nicer UI screenshots)
http://udm.adrianvintu.com/
(very nice blog post about custom dictionary locations for word, outlook and for how to edit them)
http://www.techrepublic.com/blog/msoffice/how-do-i-remove-misspelled-words-from-the-custom-dictionary-in-office-2003-or-2007/181
(Tutorial for Chrome's bdic file created using the convert_dict tool in the Chromium project)
http://www.chromium.org/developers/how-tos/editing-the-spell-checking-dictionaries

C:\Users\Daniel\AppData\Local\Google\Chrome\User Data\Default\Custom Dictionary.txt
C:\Users\Daniel\AppData\Roaming\NetBeans\7.3\private-dictionary-en_US
C:\Users\Daniel\AppData\Roaming\Microsoft\UProof\CUSTOM.DIC
C:\Notepad++\plugins\Config\Hunspell\en_US.usr
c:\Notepad++\plugins\Config\Hunspell\UserDic.dic

As a homeschooler (red squiggly line), geek, word inventor, and friend of someone who has a very uncommon name spelling, I find myself entering a lot of words into spell checker dictionaries. Compounding the problem is the fact that I use more than one application. Firefox’s spell checker uses a separate dictionary from WL Writer, which I’m pretty sure uses a separate dictionary from Word, and none of those are the dictionary used by Google Docs. It gets pretty frustrating after a while, having to enter all these words in different places.

So, my question is, does anybody know of an application that can synchronize the spell check dictionaries used by different applications? A preliminary search of Google and Lifehacker (red squiggly line) yielded no useful results. I’ll keep looking around, but if you know of any software that does this well, I’d really appreciate if you’d mention it.