In a desktop environment, Visio and Illustrator are two good options. If you use a general-purpose graphics application, you'll want to find a set of stencils that provides you with symbols appropriate to representing a web interface.
Another alternative is to use a web-based application. Some online tools are specifically dedicated to wireframing (http://www.gomockingbird.com), while others are more versatile diagramming tools (http://www.gliffy.com). Just like every other web-based option, some are free, some require subscriptions.
You can easily search your way to the following slightly dated lists, but for your convenience:
10 Completely Free Wireframing and Mockup Tools
10 Free Wireframing Tools for Designers
10 Excellent Tools for Creating Web Design Wireframes
