Below is everything from WordPress 3.0.1 found in the `parse_query()` function of **/wp-includes/query.php** `(lines 1246-1550)`.  
However, it's possible for a plugin to add parameters and so a list can never be fully complete:

 - `attachment`
 - `attachment_id`
 - `author`
 - `author_name`
 - `cat`
 - `category__and`
 - `category__in`
 - `category__not_in`
 - `category_name`
 - `comments_popup`
 - `day`
 - `error`
 - `feed`
 - `hour`
 - `m`
 - `minute`
 - `monthnum`
 - `name`
 - `order`
 - `orderby`
 - `p`
 - `page_id`
 - `page`
 - `paged`
 - `pagename`
 - `post__in`
 - `post__not_in`
 - `post_status`
 - `post_type`
 - `preview`
 - `robots`
 - `s`
 - `sentence`
 - `second`
 - `static`
 - `subpost`
 - `subpost_id`
 - `tag__and`
 - `tag__in`
 - `tag__not_in`
 - `tag_id`
 - `tag_slug__and`
 - `tag_slug__in`
 - `tag`
 - `taxonomy` - (pre 3.1)
 - `tb`
 - `term` - (pre 3.1)
 - `w`
 - `withcomments`
 - `withoutcomments`
 - `year`

 **Pre WP 3.1**
 - `meta_key`
 - `meta_value`

 **Since WP 3.1**
 - `fields`
 - `meta_query`
 - `tax_query`