There are a couple on the microsoft/mobile/pocketpc web site. I downloaded FB FltLog which has a lot of features - most of which I'll never use. Unfortunately there are a few features I'd like which are not there.
There is also a freeware Excel sheet which I have downloaded but not tried yet.
However, despite having quite a few electronic gizmos, there is something reassuring about a slightly battered, chunky log book - pity they are such a pain to carry around.