I'm a huge advocate of Yojimbo http://www.barebones.com/products/yojimbo/, the fabulous information organizer from Bare Bones software. I use it to house my research for all my writing. One tip that I found to be particularly awesome is that you can grab a link to a document in Yojimbo and link it to your NWP document.
Under the Edit menu in Yojimbo select copy item link under the Edit menu. Then in Nisus Writer Pro select some text and choose Insert > Hyperlink > Add Link... The link will open the item in Yojimbo directly.
Thanks for the tip Lou! This works with all kinds of links/URLs. You can even "Copy" a file in the Finder (which actually places a link/URL on the clipboard), and then use Nisus Writer's Insert > Hyperlink > Add Link menu to insert a link to that file (eg: clicking the inserted link will show/open the file).
