I haven't seen the Boost License being used outside of Boost, although certainly the license can be applied to any open source software. Since the Boost license is similar to the BSD style license, it is likely that an open source project would choose one of the more popular BSD or MIT style licenses.
The Boost Software License (BSL1.0) has been used by a number of other projects. The POCO project is one user: http://pocoproject.org/poco/info/license.html SourceForge has 17 projects listed as using the BSL1.0: http://sourceforge.net/softwaremap/trove_list.php?form_cat=678 (of which 2 are Boost related). I'm sure that this is not a comprehensive list. Before/during the creation of the BSL1.0, other opensource licenses were examined and found unsatisfactory. Some of this discussion is at http://www.boost.org/users/license.html , more can also be found at http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?Boost_License . If you agree with the reasons for the creation of BSL1.0, then it makes sense to use it for your projects and to prefer software that uses it. Particular things I like about the BSL1.0: 1) The single form of the BSL1.0 license means that I don't have to worry about all the minor modifications that seem to appear when people edit BSD style licenses for their project. 2) I don't have to edit the license (and worry about possible legal ramifications) or include it in every file. The short form is enough: // Copyright Joe Coder 2004 - 2006. // Distributed under the Boost Software License, Version 1.0. // (See accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt)