One of the irritating feature for me in WordPress 2.6 is the post revision. I am the only author of my blog and hence this feature is useless to me.
Just in case you are wondering how post revision works, whenever a post is edited, a new row will be created in wp_posts table. Hence if your posts or pages got edited 10 times, you will have 10 new rows in wp_posts table.
In no time your wp_posts table will be filled up and the post ID will be huge.
To turn off this feature, add this following code to wp-config.php:
You can also delete all post revisions by running this query in phpMyAdmin:
DELETE a,b,c FROM wp_posts a LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id) LEFT JOIN wp_postmeta c ON (a.ID = c.post_id) WHERE a.post_type = 'revision'
Be sure to backup your database first before performing any queries in phpMyAdmin.
*UPDATE* Auto Saves does not create a revision of the post.
*UPDATE 2* Updated SQL query from Andrei Neculau  as the previous query does not delete from wp_postmeta and wp_term_relationships tables.
*UPDATE 3* There is a proper way of cleaning up Post Revisions as mentioned by kitchin in Deleting Post Revisions: do NOT use the a,b,c JOIN code you see everywhere . I like his method more than the SQL query above.