I donít tend to use spellcheck very often nor is there any particular online dictionary I favor. I do use Merriam-Websterís Collegiate Dictionary — 11th Edition every day. Itís amazing what you can find in the dictionary.
My preferred source for grammar, style and usage questions is Words Into Type, Third Edition (Prentice Hall). Also useful is The Associated Press Stylebook and Libel Manual (ďAPĒ). Keep in mind the caveat that AP is aimed primarily at newspapers.
Likewise the Chicago Manual of Style would be your preferred source for writing a scholarly paper or for a work published by an academic publishing house. But it is not particularly useful in my business and besides, it doesnít have a good, usable index, which means itís not user-friendly.
I have shelves of style guides and manuals in my office, but besides WIT and AP, these are the ones I use most and recommend:
The Gregg Reference Manual, Ninth Edition, Willam A. Sabin (this is where to look for e.g. how to format a business letter or a bibliography)
The Compact Oxford English Dictionary (keep your magnifier handy)
The Elements of Style, Strunk & White
Modern English Usage, H.W. Fowler (just open to any place and start reading — Fowlerís hilariously dry wit and imperious tone always entertain in this 1926 tome from an era when precision in language really meant something)
I frequently use the online encyclopedia Wikipedia (<http://en.wikipedia.org/wiki/Main_Page>). Though I keep in mind that it is not absolutely unimpeachable, for most usages it will tell you what you need to know about an amazing range of subjects.
Besides Wikipedia, thereís such an explosion of online resources that itís impossible to keep track of them. Go to <http://dir.yahoo.com/Reference/dictionaries/> for a list thatíll get you started.
For help in converting American English to British English and vice versa, try any of these sites: