tinySpell

Any chance of getting tinySpell on the platform?

There is a completely free version.
There is a portable version already so most likely it would be pretty easy to adapt to portableapps.
It seems like there is no equivalent or similar function on portableapps now.

It's not open source, but I understand that is no longer a requirement.

http://tinyspell.numerit.com

Description: "Occasionally you need to check spelling in an application that does not include a spelling checker and you don't want to launch your word processor just for that. This is when tinySpell becomes handy. It is a small utility that allows you to easily and quickly check and correct the spelling in any Windows application. tinySpell can watch your typing on the fly and alert you whenever it detects a misspelled word.
It can also check the spelling of text that you copy to the clipboard. tinySpell installs itself in the taskbar notification area for easy access."