Depends what you want.
Logbook Pro, which is heavily advertised on this site. Is excellent. I don't use it (I created my own), so this isn't a plug. It's powerful, flexible, customizable and appears to use good database architecture.
You'll also find, if you search around, a number of home-grown free logbooks, mostly in Excel. Most are simple spreadsheet versions of the paper log, but I've come across some that are really sophisticated.