The Hack Open Font License is a weird mix of the Bitstream Vera and SIL Open Font licenses. It has the common "must be renamed if modified" and "must not be sold by itself" clauses that are found in font licenses. These clauses might make a software license non-free (though, the first one seems to be acceptable by the FSF in some cases), but they have been considered acceptable for FOSS font licensing for some time now. It is worth noting that the Hack font upstream (the origination of this license) feels that the "sold by itself" clause can be trivially circumvented and has no issue with people doing that (e.g. font comes with a helloworld binary).
Hack Open Font License v2.0 (Version 1.0 - 06 September 2015) (Version 2.0 - 27 September 2015) Copyright 2015 by Christopher Simpkins. All Rights Reserved. DEFINITIONS "Author" refers to any designer, engineer, programmer, technical writer or other person who contributed to the Font Software. PERMISSION AND CONDITIONS Permission is hereby granted, free of charge, to any person obtaining a copy of the fonts accompanying this license ("Fonts") and associated source code, documentation, and binary files (the "Font Software"), to reproduce and distribute the modifications to the Bitstream Vera Font Software, including without limitation the rights to use, study, copy, merge, embed, modify, redistribute, and/or sell modified or unmodified copies of the Font Software, and to permit persons to whom the Font Software is furnished to do so, subject to the following conditions: (1) The above copyright notice and this permission notice shall be included in all modified and unmodified copies of the Font Software typefaces. These notices can be included either as stand-alone text files, human-readable headers or in the appropriate machine-readable metadata fields within text or binary files as long as those fields can be easily viewed by the user. (2) The Font Software may be modified, altered, or added to, and in particular the designs of glyphs or characters in the Fonts may be modified and additional glyphs or characters may be added to the Fonts, only if the fonts are renamed to names not containing the word "Hack". (3) Neither the Font Software nor any of its individual components, in original or modified versions, may be sold by itself. TERMINATION This license becomes null and void if any of the above conditions are not met. THE FONT SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF COPYRIGHT, PATENT, TRADEMARK, OR OTHER RIGHT. IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, INCLUDING ANY GENERAL, SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF THE USE OR INABILITY TO USE THE FONT SOFTWARE OR FROM OTHER DEALINGS IN THE FONT SOFTWARE. Except as contained in this notice, the names of Christopher Simpkins and the Author(s) of the Font Software shall not be used to promote, endorse or advertise any modified version, except to acknowledge the contribution(s) of Christopher Simpkins and the Author(s) or with their explicit written permission. For further information, contact: chris at sourcefoundry dot org.