I tried whitesmoke because I wanted to review it for the course section of my website and I have to say I was dissappointed.
It made very strange suggestions that didn't fit at ALL with what I was writing. Stick with Mircrosoft word and the grammar/spell checker. That is just as good and cheeper.
As for finding somethinf to proofread your work for a low cost it is hard to find. A program can't do it as well as a real person.
Some of you might be interested in my writing course that is on my website. Each week I post a new assignment which you can submit a small essay for and I correct and post it on the site ASAP. This is all for FREE of course. I'm in the buisness of helping people learn English. For for info visit the course page at (URL removed by mod).
Have a great day!