Recomendation for a sticky-note software with a "connection" function?

I have a bunch of ideas for my story and I need to write them down for brainstorming, since I can no longer hold them in my head (my memory is getting full, beside holding too many ideas in my head cannot help me brainstorm effectively). A Fluther member once brought forward the suggestion of writing ideas on sticky notes. I like the suggestion but I don’t want to use real sticky notes and write my ideas on them then stick them onto a wall, it’s kind of messy. I need a sticky-note software with a “connection” function (by saying “connection” I mean I can make sticky notes then connect related ideas together for easier analyzing). Do anybody here know any software like that?