I have spellchecker in almost every application.

Mozilla Thunderbird and Mozilla Firefox:
have a very good way to deal with spell checking. Inside a right click menu you can chose the language for each window or mail. Thought the right click menu you can also enable and disable this status. If you download the English version of Thunderbird/Firefox then you get the English dictionary by default, with German vice versa. However, you can download any distribution and still download and install new dictionary's easy thought addons.

Pidgin (before known as Gaim):
a popular multi protocol instant messenger can also support spell checking if you install aspell.

So far about the Open Source (GPL) programs.

Opera:
a well known browser also supports spell checking, you just need to download aspell yourself. Opera is Closed Source and proprietary and it`s still no problem to download aspell yourself, them prepared everything you need.

Every of this application works very well. When you write a wrong word then it is underlined red, you click right on it and choose the correct word.

And... It does not lag and it`s not slow. So that "that would hang mirc" is really really really out of the window.

So far about the technical possibility's.

Is there some third party script out which implements spell checking? I would really need that for mirc.