Erik Swenson has published a post showing his phenomenal collection of wireframes for SharePoint 2010. I remember when he showed me his SharePoint 2007 wireframes in Montreal last year and being blown away.
Erik’s approach is to go super Hi-Fi. By that I mean that he creates exact, faithful reproductions of the SharePoint interface in Visio. But, since Erik is not allowed to share these widely, you may want to consider another approach: Instead of creating Hi-Fi wireframes (and investing the incredible amount of work required), try the Lo-Fi approach.
I prefer this course of action, because, depending on the phase of the project, I don’t want clients to think too much about how it will ‘actually look’, but more about ‘what function/element belongs where’.
The tool I use is super fast and easy (and cheap), and you can even use it interactively during client workshops. It is called Balsamiq Mockups (www.balsamiq.com).
And, just when you think this can’t possibly get any easier, along comes Gordon MacLeod (a fellow Torontonian) who has created a bunch of pre-built SharePoint elements that you can download for free from here: http://mockupstogo.net/prebuilt-sharepoint-elements.
I have heard great arguments for both the Lo-Fi and Hi-Fi approaches, and they both have their uses. For me, fast, schematic and interactive wins out.