Jabber \ XMPP Clients

While not comprehensive, here are some clients our friends and us have used over the years.

Pidgin		Pidgin is a cross-platform open source instant messaging client. It supports XMPP, so it will work with Subspace.IM, but it also supports other protocols such as Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, Lotus Sametime, SILC, SIMPLE, and Zephyr. Pidgin is a very plain & simple "get it done" kind of IM client.
Adium		Adium is an XMPP client for Apple Mac OS. It is an open source client based on libpurple, so can be regarded as the Pidgin equivilant for Mac. Adium supports a wide range of IM services including XMPP (Jabber), ICQ, IRC and more. It also supports Mac OS address book integration and WebKit message view.
Monal		Monal is an XMPP client for Apple's iOS devices. Monal is compatible with both the iPhone and the iPad, and has a tablet layout which is sensible for the iPad. Monal is one of the few, if not the only, XMPP client available for Apple devices which does not use a 3rd party relay server that could potentially compromise your account security. Monal has some rough edges (for example, the contact list won't appear if it's opened in landscape mode on the iPad), but it's really coming along nicely, and it's maintained by security focused developers. Monal is free on the App Store.
Xabber		Xabber is an XMPP client for Android devices. Xabber does not make use of a third party relay server that could potentially compromise your security. Xabber is free on Google Play and the Amazon Appstore. Xabber is ad-free, supports multiple accounts, and is highly customizable.
Spark		Spark is an open source XMPP client for Windows, Mac, and Linux. Spark offers a fantastic end-user experience complete with inline spell checking, tabbed conversations, and more.
Miranda		Miranda is an open source instant messaging client for Windows with support for multiple protocols including XMPP. In addition to XMPP, Miranda supports Discord, Facebook, ICQ, IRC, Steam, and more, making it a great stepping stone client. Miranda is built with simplicity in mind, so you won't be getting a lot of extra eye candy. Miranda is one of the most lightweight IM clients available for Windows.