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 ($25).
 User Interface
Pidgin uses GTK+ and uses a very familiar, "regular application" appearance. Trillian more unique, polished user interface. 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, on Windows (the only platform Trillian is available on, other than Mac OS X since lately), is massive and uses many dll's compared to Trillian. Of course this has no relevance to size or memory footprint
Pidgin is faster than Trillian. The latest version of Trillian now has constant popups and balloon messages unless you pay for an annual registration. There are regular updates to Trillian, adding new features (Facebook chat is available on Trillian, though it is also available on Pidgin).
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.) Unlike Trillian, which requires host files and executable files to be on the same disk.
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 Trillian Pro both provide plugin support. Trillian Basic does not. Probably because of social factors related to its open source development model, Pidgin enjoys a far greater selection of available plugins than Trillian Pro, however.
 Video chat
Pidgin supports audio chat for XMPP/Jabber only.
 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.