I haven't needed to spec out a website for the past couple of years, but I'd wish I'd had Mockingbird then.
Web designers and application designers should definitely check out Mockingbird for wireframe development. Essentially, it's an online tool for sharing basic layout.
Prices range from $9-$85 monthly, depending on the number of active projects you need. With all of their paid plans you can use unlimited collaborators and archive an unlimited number of projects.
Another reason I like Mockingbird is that they only charge you based on usage. If you have no active projects, you aren't billed, which is a refreshing change from most web services. It's great for the small designer who is working independently on a project or two. Bigger design firms will have no problem paying the monthly fees, because again it's based on the number of clients. If you can't squeeze $40 across 25 clients, you aren't doing something right.
When you're done with the project, you can archive it. Need to do updates? Simply move it out of the archive to the active projects again. Just don't forget to archive them when you're finished because you're billed based on active projects.
Like I said, I've just briefly played with Mockingbird, but it seems like a useful development tool. If you have more experience, positive or negative, with wireframe development, please let us know in the comments. Also, let me know if you run across another tool that's different or better.