We are all used to spell checkers in our office suits, but what about the texts we write in other applications? Why should those be considered not important enough to be spell checked? E-mails, forum and other posts from the browser, conversations over instant messaging services and all the other texts that are not made in office suits like code and documentation for example. Well, Aspell is here to change that!
Aspell is considered the standard GNU/Linux spell checker and is thus used by a vast number of projects/applications. Amongst more notable KDE, where it can be seen almost everywhere. From the Kmail e-mail client, the plethora of text editors even to the browser Konqueror and the IM program Kopete. All you need to do is have Aspell installed together with the dictionaries you need and in Kcontrol set the "KDE Components → Spell checking" to "Aspell" and the dictionary you want to use as default. After that Aspell already kicks in and starts checking your spelling.
I have known about this for quite some time, but didn't have it enabled because it used to be very slow. But luckily things change and the awesomeness of FOSS wins again and (KDE's integration of) Aspell developed to very fast.
Also of note should be that Aspell is neither the first nor the only spell checker for GNU/Linux. It's predecessor from UNIX is Ispell, but Aspell has a better support for UTF-8 and can work with many different languages at the same time, to name just a few of its features over Ispell. Probably even more important is that Aspell is still being developed further and that the (main) developer is still taking care of it. Then there is also the Hebrew Hspell, which is an obvious choice for those who need Hebrew spell checking. In any case there is no problem for all of them to co-exist on the same system.
And for the last word: It is still unwise to fully trust any spell checker – it's just a tool that suggests what it considers to be written wrongly and not someone with a PhD in grammatic and wordbuilding.