With all of the different sites, forums, blogs, etc… that use textareas for writing on the web, it’s unrealistic to expect them all to have a good spellchecker. Even sites that do have spellcheckers have very different levels of quality. I finally had the common sense to install a browser based spellchecker more recently and what a difference!
Spellchecking doesn’t belong on websites because it requires significant personalization to work well depending on the authors areas of interest or expertise. For example, I want to tell my spellchecker to ignore springmvc, webwork, mysql, postgres, etc… Given most people don’t run browser based spellcheckers at the moment it’s still a good idea to have one built into your site but I hope that need goes away soon. The most popular browser options at the moment are using the built-in feature in Safari, the Google toolbar, Spellbound for Firefox (doesn’t support 1.5), or IESpell for IE.
The bottom line is that if you want good reliable spellchecking you need to have it in your own browser. My hope is that browsers begin shipping with built-in spellcheckers turned on by default that spellcheck as you write (Safari already has it and it looks like Firefox will be getting built-in spell as you type in the 1.8 branch). Then we can finally forget about having to integrate spellchecker X, Y, or Z into our sites and get on with it!
Related reading: Spellcheck as you type built into Safari, Mitchelaneous on SpellBound, Spellcheck as you type comes to Firefox, Harry Chen on upgrading WordPress, The best spellchecker for WordPress, In love with Spellbound