Pidgin vs Trillian
Pidgin and Trillian are multi-protocol instant messaging clients. Pidgin is free software, licensed under the GNU General Public License (GPL) version 2. Trillian is closed source and comes in two versions: Basic (free) and Pro ($20 a year or $60 a lifetime).
 User Interface
Pidgin uses GTK+ and uses a very familiar, integrated appearance. Trillian more unique, Windows 7 like polished user interface. Trillian features api's for skinning, although Pidgen my be considered more in depth. Some users prefer the more flashy and customized appearance of Trillian, while others the less obtrusive, and for most operating systems more visually integrated, appearance of Pidgin.
Pidgin's program files folder on Windows is approximately 52 megabytes when installed compared to Trillian's 70 megabyte folder.
Pidgin starts up faster, and often responds faster than Trillian. The latest version of Trillian has occasional popups and defined in line ads for free users. There are regular updates to Trillian, adding new features.
Pidgin has been ported to many operating systems, including BSD Unix, Linux, and MS Windows systems. While the Pidgin application itself has not been ported to MacOS X at this time, the libpurple library -- which provides its core functionality -- has, and is in fact the basis of the MacOS X multiprotocol IM client called Adium. - Pidgin can also be safely installed on any flash drive, allowing it to be used as a mobile IM for people either on the go, or public computers where installing software is not allowed. (Such as the library or a school.) Trillian, requires host files and executable files, as well as well as chat history storage in app data. Trillian has been ported to many mediums such as Mac, Android, iOS (one for iPhone and iPod and a separate version for iPad), Blackberry, and a build for debian based linux. History will sync across installed Trillian clients for pro users.
While both Pidgin and Trillian support encryption, the free Trillian Basic client does not support end-to-end OTR encryption. The paid-for Trillian Pro and open source Pidgin clients both support OTR via a plugin.
Pidgin and all versions of Trillian both provide plugin support. Probably because of social factors related to its open source development model, Pidgin enjoys a far greater selection of available plugins than Trillian, however.
 Video chat
Pidgin supports audio chat for XMPP/Jabber only. Trillian supports video chat for its own im service Astra, as well as audio support for XMPP/Jabber and skype (with the use of skypekit).
 Cloud-based History
Trillian Pro saves message history on their servers so that it can be available to be read on other devices that use Trillian Pro with the same membership.