Mudlet

First of all, I'm not that good at deciding whether it's legal to "portableize" an application. But I'll give it a go. If I'm wrong at some point, just tell me mods!

Second: check the site, before you come and ask.
Hence the red text, They even enourage to send them an email.
I know you arent going to modify the source code and redistribute it, you're simply going to make it portable.
So please if you really want to know, email them.

Mudlet is opensource, under the Creative Commons 3.0. There is no need to ask the author for permission, as the license itself allows you to "share" and "remix" the contents licensed, meaning that you can freely redistribute it packaged in portableapps.com format.