Great program, but be careful with the user-contributed lexica. The JLPT-4 lexicon appears to have some Kanji errors. I'd suggest having a native Japanese speaker check before you use this lexicon. Otherwise, you may end up learning things you will have to correct later.

Example: find the word for "skilled" in the lexicon. You'll see the lexicon creator has substituted the Kanji for "un" in place of the Kanji for "upper." The same problem carries through with other words that appear to confuse "un" (fu) with "upper" (ue).

Fortunately, the Kotoba program allows you to correct the lexicon.