Direct Connect

ApexDC++
Presents this open source client based on the StrongDC++ modification and which supports, amongst other features, PeerGuardian IP blocking and swarming. Also offers language packages, toolbars as well as emoticon packages for advanced customization.
http://www.apexdc.net/

DC++
Open-source client written in C++, ad-free.
http://dcplusplus.sourceforge.net/

Dcsharp
Open source Direct Connect client for GNU/Linux. It uses the GTK toolkit and its goal is to be as easy to use as possible.
http://code.google.com/p/dcsharp/

DCTC
Open source command line client for GNU/Linux. A GUI for the client and a hub program, both in beta stage, are also provided but the project is no longer maintained.
http://ac2i.homelinux.com/dctc/

HubTracker.com
Provides a hublist for NMDC and ADC protocols as well as automatically updated statistics about listed hubs.
http://www.hubtracker.com/

Linux DC++
A port of DC++ to GNU/Linux and to any POSIX-compliant UNIX. It has the same core with a rewritten user interface. Site includes documentation, FAQs and screen shots.
http://linuxdcpp.berlios.de/

RevConnect
Open source client derived from DC++ with additional features such as Kademlia and multiple source downloading.
http://www.revconnect.com/

ShakesPeer
Presents this open source client for Mac OS X, includes a support forum and an RSS feed for those who'd like to track development information.
http://shakespeer.bzero.se/

Valknut
Open source, cross-platform client for the old NMDC protocol. It doesn't support the new ADC protocol developed by the DC++ team.
http://wxdcgui.sourceforge.net/