Client

DefBot
A very customizable automated Hotline client for server administration and user moderation.
http://www.s-and-m-software.com/

Fidelio
Open source GNU/Linux client that supports most of the latest Hotline features.
http://fidelio.sourceforge.net/

GtkHX
A Hotline client written for Unix/Linux-like operating systems. It employs the Gtk+ environment for the front-end and utilizes a back-end based on the HX project.
http://gtkhx.sourceforge.net

Hotline Client How-To Guide
Offers an explanation of the way the official client works, also includes a FAQ on connection errors and navigation tips. Apparently no longer maintained but still of interest.
http://www.hl123.com/hotline/

Hotline-Tools
Shareware Macintosh client and utilities for Hotline users.
http://www.hotline-tools.com/

HotPants
Open source Hotline 1.2.3-compatible client for Macintosh OS X.
http://homepage.mac.com/haveblue/hotpants/

JHL
A Java-based Hotline client.
http://jhl.johnl.org

Monica
A shareware Macintosh download utility for the automated downloading of files from the Internet using popular file transfer protocols, including the Hotline protocol.
http://www.blackdiamond.co.za/bdmonica.html

SilverWing
A Hotline client for BeOS. A version is available for the x86 hardware platform as well as PPC.
http://hp.vector.co.jp/authors/VA013465/silverwing-beos/

Underline
A variant of the OpenLine client for MacOS and Windows that supports blowfish encryption. Licensed under GNU GPL.
http://www.hyperspasm.com/underline/