The information below came to me via the listserv: web4lib, which quotes CNET.  

The Opera Web Browser software is known for NOT being built on the Mosaic model (like Netscape Navigator and MS IE).  Folks who use it say they like it because it runs fast and requires a less-robust computer than the more popular browsers.  Until now it has cost money.  Now it is being offered free with ads, or without ads for a $39.   (The latest version of Eudora email software also uses this model.)

Betty Brown
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[Description below from CNET]

Opera is a user-friendly, versatile, and fast Internet/intranet browser 
that also includes a newsreader. The keyboard plays an integral role in 
surfing, which can make moving around the Web easy and fast. You can run 
multiple windows, even at start-up, and special features are included for 
users with disabilities. Opera supports CSS, Java (through the Sun Java 
plug-in - for the Java-enabled version click here), JavaScript SSL 
(128-bit), TLS (Transaction Layer Security), and more.
Other options include the ability to design your own look for the buttons 
in the program; file uploading support to be used with forms and mail; an 
option to turn off tables; advanced cookie filtering; and a host of other 
powerful features, including an email client enhancement, keyboard 
shortcuts, and more.
New functionality in the 5.0 release includes:
       Integrated search and Instant Messaging
       Support for new JavaScript versions
       Updated email support
       Improvements in memory usage
       Accessibility to different Web sites, and more
Note: The full version of Opera is now available for free in an 
ad-sponsored version. Registered users will not see ads. The registration 
cost is $39. The upgrade from Opera 4.0 for Windows is free.