I use Google Docs, which has a pretty good grammar and spellcheck feature. Before I publish a chapter, I copy/paste it into Microsoft Word just to see if there's anything a different program catches. Then I do a read through looking for errors that the algorithm struggle with, (like when I accidentally write "ad" instead "as" for the thousanth time). 

Last I do a read-through for how the writing flows. If I find myself skimming, I know I wrote something boring. If I find myself hitching on a description or sentence structure, I rework that section.

Nothing beats a real editor, but when I don't have one this works well enough.