I have tried several wire framing tools over the last 9 months. My first taste was with Microsoft Visio, then onto Axure. Both are great for creating wire frames. Axure has the upper hand here as it allows you to make your wire frames interactive.

This can also be said for Adobe Fireworks, however this isn't purpose built just to do wire framing, so if you already use Photoshop then better to stick to what you know.

As for online alternatives, I have tried HotGloo for the past few months, it was free, but now comes at a price.

Since HotGloo's announcement for paid accounts I started looking at Pencil Project for Firefox. Its a plugin that uses the Firefox engine. It's Free and certainly worth a shot.

I've heard about Mockingbird.  It's an online solution.