On of the more extensive IM client out there is Miranda. I haven't covered this one yet but I figured should considering its portable right out of the box. One thing I like is the detailed customization of the client. Allowing you to not only change appearance, but how you you manage your contacts, status, the list goes on.
I first played around with it back in 2005 looking for an alternative to Trillian. And while it looked portable, paths in the application were stored in the database as absolute instead of relative. However there was an add-on that performed this very task. Although it was made obsolete a few updates afterwards. One of the things that makes Miranda stand out is the community of people contributing add-ons that either an in a new protocol or service or even change the way it looks or stores information.
With the new 0.7 release I can admit I am impressed and recommend this to anyone still looking for something different.
The release you want to grab in the zipped release from the Sourceforge page found in the downloads section. There will be a Zip file and that is what you want to grab. After unpacking you'll either want to remove the "mirandaboot.ini" file found in the miranda directory or just change the follwing line:
By changing the path to whatever you think is best, and the default is the Miranda directory if you leave it blank. And there you have it...Miranda portable without having to perform massive surgery.