Windows only: Freeware application Enso Words adds an always-accessible universal spell checker and dictionary to Windows. Enso Words is very similar to the much-maligned (by me, mainly) Enso Launcher, but focuses on, obviously enough, words. Highlight any text on your computer, invoke Enso Words, and type the spellcheck command for a quick spell check and autosuggest corrections. Enso Words also integrates with web searches at Answers.com to provide definitions or synonyms of either selected words or words you type into the define command.
For other universal spellcheck alternatives, check out tinySpell and CleverKeys.
