The X-windows version is easier to use, because most of the functions are on menus. Another drawback is that it's mostly text based. It is also free, and the source code is available under the GNU public license. NCSA spent four years on the project and has now moved on to other things. The latest version is 2.6 which was released on July 7, 1995.
Source code is available for non-commercial use.
Spyglass Inc. has the commercial rights to Mosaic. Its a solid X-windows browser, but it lacks the new HTML features. Hence it supports all the HTML 3.0 standards such as style sheets and tables. Development was taken over by Yggdrasil Computing, with the idea to turn it into a full fledge free X-windows browser. However development has stopped in Feb 1997 with version 0.3.11.