I really like Fireworks. It's similar to Photoshop and also by Adobe, but is more suited for website mockup and mobile designs.
If you look at their templates (click File->New From Template) there are wireframes for jquery, iphone, ipad, and some sample web design layouts. They also have a Common Library with menus and design elements that you can drag and drop into place.
You can get a month free right now with their Adobe CS6 Cloud offer. Hate sounding like an advertisement, but it's worth checking out if you want the final wireframe to look like a real website.
This is a wireframe I'm working on for an application I'm writing. I'm more of a programmer than designer. I may use this just to measure pixel widths. It may end up as a WPF app... I'm not sure, but this has been a great way to get started.