|This page documents an English Wikipedia editing guideline. It is a generally accepted standard that editors should attempt to follow, though it is best treated with common sense, and occasional exceptions may apply. Any substantive edit to this page should reflect consensus. When in doubt, discuss first on the talk page.|
Be careful about setting the language of the text to the right dialect of English. (See Wikipedia:Manual of Style#National varieties of English.) If the article is not about a topic tied to a particular region, check in which dialect of English the first non-stub version of the article was written. This can require a bit of effort, depending on the presence of "dialect marker" words. If you see spellings like "color" or "center" in the first non-stub version, the spelling was almost certainly American (though it might have been Western Canadian). Spellings like "criticise" mean the article was definitely not written in American English, and very likely not in Canadian English, but determination of which of the Commonwealth dialects of English it was may be difficult. However, the differences among the Commonwealth dialects, especially those outside of Canada, are very small. Note: some Wikipedians claim that if the dialect of an article has "drifted" from its first non-stub version, via a series of unmotivated spelling changes, then the dialect to which the article has drifted should be considered the "correct" dialect of the article.
Using Microsoft Office or OpenOffice.org 
The default setting in Microsoft Office is with spell and grammar checking on, so just hit the Wikipedia "Edit" button for your article, copy the raw article source, paste it into a new Word or Writer document and follow the red (spelling) and green (grammar) markers and correct mistakes as necessary. If checking is not automatic, you may have to go to the menu Tools -> "Grammar and spell checking" or some such. When satisfied, simply copy and paste the text back into the text box in the browser window, check that any UTF-8 characters are still working as before by clicking "Show changes" and if satisfied, finally click "Save page" as usual.
Using a web browser 
Mozilla Firefox has a built-in spell checking engine since version 2.0. However, you should check that the right dictionary for your language is installed and selected. Right-click in an editing field, make sure that "Check spelling" is checked, then under "Languages" select the right language. There are separate dictionaries for "English / United States", "English / United Kingdom" etc.; if the right dictionary does not appear in the list, click "Add Dictionaries" and use the page that appears to install it (note that you will have to restart Firefox to enable it after installing).
Safari also provides spell checking features on both Mac OS X and Windows versions.
SpellChecker.org is a free online spell checker that requires no extension or add-on installation.
On Mac OS X, Opera uses the system spell checker. There is no need to install additional software. On Windows, Linux, and UNIX, Opera will do spell-checking if both GNU Aspell and at least one Aspell dictionary are installed.
Live spellcheck 
If you have installed the anti-vandal tool in your monobook.js, you can use the live spellcheck to identify misspellings as they happen (recent edits). This is not a comprehensive spell check – it only points out the most commonly misspelt words. And it will not spellcheck text that an editor is adding to an article (for that, see the web browsers section, above).