Mockups are a key part of most tech and online ventures, especially those featuring software, web sites, and web services.  They succinctly present what you do and what makes it valuable.  The user interface can make or break your success.  People are visual.  Investors, partners, management, developers, and early users all need to see your idea to understand it.  Whether you're pitching your concept in the early days, prototyping, or developing specs for your programmers, you need to quickly and effectively develop mockups, including wireframes, flow diagrams, and prototypes.

Online mockup apps or resources

• Axure RP. 
• Balsamiq - $79 app for wireframes
• Draftium - free and $99/yr levels
• Keynotopia - tutorials, examples, and user interface templates for prototyping 
• Pencil Project - free Mozilla add-in or standalone app for diagramming and prototyping
• Creately - collaborative diagrams, $5/mo with team options
• Prezi -  web service for infinite whiteboarding and cool animation for shows, free for public or educational use, $60/yr for private us
• Dia - Diagramming software comparable to Visio

Old school. Other tools that can be used to create mockups

• Powerpoint or similar
• Graphic programs
• HTML and web site generators