It looks like there are a couple: Captains's Log and
ED Discovery, with Captain's Log suggesting that it has the advantage of storing your logs in a database that Frontier won't decide it might erase and maybe storing more data on planets (?), and EDDiscovery integrating your data online, which seems like a gesture of good citizenship. I'm thinking it would have been nice to run one before I started off.
EDD stores all your history and journals in a local database, just like Captain's Log. The online data also provides a backup, EDD will repopulate from EDSM if you lose all your journals and the local database.
Providing the option to sync local data with the online databases (EDSM and EDDN) is on Genar's to-do list for Captain's Log, and the graphical representation of system scans with material data is getting a lot of love in EDD right now.
Both will load all your logs from when (before) you set out if you install them now, so all you've actually lost is entering notes as you go.
You can run them both, the only real reason to pick one is subjective preference.
Unless you're the sort who likes to jump into the code and add new features if you think of something cool and then EDD is the open source one.