Here are some spelling checkers to assist you in your Wikipedia editing. Please spell check before you save an article. (Also nice for spot checking articles).

• Mozilla Firefox add-on for U.S English, also British, Canadian, Australian and South African.
• Google Toolbar offers spell checking in various languages for Internet Explorer.
• A well-designed spell checker for Internet Explorer is ieSpell.
• For Mozilla Firefox textboxes, there's SpellBound.
• Konqueror uses Aspell if it's installed and set up in your KDE Control Center.
• A good stand-alone spell checker for English Wikipedia pages is Inspyder InSite. (The free version is good enough for fixing Wikipedia articles.)
• The macOS universal spell checker is available in Wikipedia's "edit this page" mode while using Safari and in theory any other Cocoa-based browser.