Using a web-based software means you don't have to worry about downloads or upgrades.

A website wireframe is a basic visual guide used in interface design to suggest the structure of a website and relationships between its pages. A webpage wireframe is a similar illustration of the layout of fundamental elements in the interface. Typically, wireframes are completed before any artwork is developed. (via)

Here are my favorite online wireframe web apps for you to try out!

HotGloo

Description: One of the strongest features of german based HotGloo is the simplicity of the alignment of the different objects on the stage. It is almost like a stripped-down version of the Smart Guides feature in Photoshop, giving what you need, without the extra lines.

Link: http://hello.hotgloo.com/

MockFlow

Description: MockFlow wants to help to enhance your planning process by enabling to quickly design and share interactive User Interfaces mockups.

Link: http://mockflow.com/

iPlotz

Description: iPlotz offers a adobe flex based web tool for quickly sketching wireframes of web sites and demonstrating navigation between pages.

Link: http://www.iplotz.com/

Lovely Charts

Description: With Lovely Charts, a new online diagramming application, you can draw Flowcharts, Sitemaps, Network Diagrams and Wireframes very fast and easy.

Link: http://lovelycharts.com/

gliffly

Description: With Gliffy you can easily create professional-looking flowcharts, diagrams, floor plans, technical drawings, and more...

Link: http://www.gliffy.com/

So have fun prototyping your next application design ;)