Asterisk does: https://wiki.asterisk.org/wiki/pages/viewpage.action?pageId=30278158 They have a good overview page

Kamillio does: https://kamailio.org/docs/modules/3.2.x/modules_k/rls.html

Good info from Grandsteam about why to use this: http://www.grandstream.com/sites/default/files/Resources/GXP21x0_Eventlist_BLF_Guide.pdf

Same with Polycom: http://community.polycom.com/polycom/attachments/polycom/VoIP/19112/1/Technical%20Brief%20-%20Busy%20Lamp%20Field.pdf

Others have asked this question on the freeswitch users list, but responses were limited.