This time last week, no KML editors in English. Today, there are two:
KMLEditor, the Japanese language editor profiled a few days ago, now has English localization. Download it here.
NorthGates’ KML Editor is a brand new entry, which you can download here.
Check out both. KMLEditor seems to have more features — for example, support for time — but it is early days yet. Anyone up for making a Mac version? (Via comments)