The the minimalist Helvetireader style from http://www.helvetireader.com/ . See that site for screenshots and further information. Note that since the CSS has been copied into this style, you will not automatically get updates from helvetireader.com.

NB I didn't come up with this - see http://hicksdesign.co.uk/journal/heres-helvetireader . You can piece together the information required from the site above; I just thought it was worth sharing via userstyles to save other people doing the same.

Works for google.com and google.co.uk, you may need to edit this for use in other regions.
[Feb 11th] Updated to latest CSS, just a few !importants added.
Updates to Helvetireader are posted on http://twitter.com/helvetireader, I usually update this userstyle a day or two later and post to http://twitter.com/bazzargh