I have worn out the search engine trying to find an answer to this question  and I think i have seen it here before, alas the search  feature seems to be a little week given the amount of content we have here. So...

I have a module with a Treeview that pulls data when a Leaf is selected and it needs to tell another module on the page to fill the information based on the selection(yes we have consider making one big module and may still do it, but the treeview module has many functions we want to reuse). How do i pass information between the modules so i can populate the desired data?

I'm only giving my $0.02 cents here because I have seen this question so much over the years.

If you want to reuse the TreeView perhaps you can include it as a reusable .ascx control on the page?

I always encourage developer's to make "small modules" and leverage the DotNetNuke framework, however, there are times where 2 pieces are functionality are really one.

For example the tree view and the resulting click comprise one functional thing. One would be useless without the other. In this case trying to make a separate module out of each part can cause needless complexity.