My neighbors husband just died. What can I say or do?

My neighbor just lost her husband last night in a motorcycle accident. She is only in her early thirtys with three children to take care of. Her middle son plays with our son. We don't know her that well but my heart goes out to her and as her neighbor I feel as though I should do something. I don't want to bother her as she must be overwhelmed with grief right now. I thought about leaving her a written note with my condolences and letting her know that I will help her if ever needed. Is that the best thing? How long should I wait to leave the note? I just don't want to pester her, but I want her to know that I care. Please, I need your suggestions.