Search engines are nice, but if you're off-line (or in need of real spell-checking) you can always start up your favorite word processor (such as Microsoft Word), and type whatever word it is you want checked. If it's a decent piece of software, it'll have a built-in spell checker.
Of course, it won't show you any of the wrong ways to spell the word (like Altavista or Google will) but it (in this case, Word) will do grammar checking as well, plus Japanese consistency checking, which comes in handy at times (for me anyway).
Also, it (the spell checker) will distinguish between different languages, as well as US English and UK English, unlike the search engine, which will consider either spelling correct. Not all people bother with trying to avoid mixing US and UK English, though.
It should be noted that while Word (the word processor I used in the example above) is according to some people a piece of bloatware, starting it up is actually faster on my system than opening Internet Explorer and clicking the link to Google, and then waiting a split second for Google to load.