LevelDB is an open-source on-disk key-value store written by Google and used in Google Chrome. It's faster than SQLite, but its files can get corrupted more easily.
A sample file can be found at
~/.config/chromium/Default/Local Extension Settings/pafkbggdmjlpgkdkcbjmhmfcdpncadgh/000459.ldb or similar.
How to have a look at the content of such files, without having to write a program?
A browser would be great, but just dumping all data is already enough.
There is no official client, just official client libraries.