A really great little spell checker that has a more powerful dictionary than Word is available 100% free. This is freeware, not shareware so it is not going to time out on you. Once you install it on your computer, all programs like this forum and almost every other program that are not MS based can use it. It only spell checks. Sorry, it can not improve your grammar.
Once installed, a small "abc" icon will appear above the smiles on this forum. I use the context sensitive menu by right clicking the mouse and then selecting > Spell Checking.
Here is a link to the download page. I highly recommend this program.
ieSpell - Download