It is essential that your customers know that they can find out more about your products and services by visiting your web site. So placing your URL in as many prominent places possible helps increase the traffic to your web site. Please find twenty-five prime locations we identified. If you can think of others, please let us know in the comments:

Southwest(.com)
Creative Commons License photo credit: quinn.anya

Business Cards
Email signature panel
Company letterhead, envelopes and other stationery
Google, Yahoo and Bing local
Your Facebook, Twitter and other social media sites
Product Packaging
Marketing Brochures
Signage (Windows, Vehicles, Sidewalk, Sandwich Boards)
Direct mail letters and envelopes
Advertising (Yellow pages & other)
TV Advertising
Client Surveys
Media Kits
Giveaways & Premiums
Address Stamps
Flyers
Product displays, for retail or trade
Trade show brochures
Tent cards
Postcards, or direct response cards
Entry forms, for contests or sweepstakes
Receipts and invoices
Fax cover page
Radio advertising, if your URL is short and memorable
Company vehicles