CaptionBot by Microsoft - Analyzing and describing images by a bot
Caption - Open-source captioning tool to help find the right subtitles.
Instagram Caption Writer - Write nice formatted Instagram captions.
Captioned - Say, show, share. Create a 60 second captioned video.
WTF is that? - Send any picture and it will tell you what it is!
Caption This - Create a smart caption for your photo in a second!