In my win forms c# app, I want to be able to add spell check functionality. But, I want to do it my own way, and NOT use any other components. All I need is just an online service where i can send a request or something and it tells me whether or not the word i sent was spelled correctly.
Preferred but not required: The ability for that online sevrice to also send back suggestions.
A quick google search turned up this:
Search Web Services: Spelling Suggestion
This is not c#/.NET specific. Its a simple REST service provided by Yahoo!. It looks like they do limit you to 5,000 queries per day for free and I'm not sure if Yahoo! lets you upgrade to a pay service.
Personally, I wonder why you want only online access...what if you lose connection? Also, most online accesses come with restrictions.. which you might hit sooner than you think during debugging.
If you were to use WPF, I'd suggest using the SpellCheck Class.
Since you specified Windows.Forms, you might try the NetSpell library for offline access. You can also try and use Microsoft Word's spellchecker, but you might not have it installed on the machine (or use Linux & Mono)
If you persist on using online access, you can use Yahoo, as Giovanni Galbo says. It's been done in C# before.