Opera recently held a UserJS competition, where I participated with a script that makes Google Spreadsheets work in Opera, a script which improves Apache directory listings and a social bookmarking site toolbar.
Opera chose my Google Spreadsheets script as the winner of Site Fixes category, and I will be receiving a shiny new HTC Touch Diamond phone with Opera 9.5 preinstalled! :D
Head to UserJS contest winners post at myopera to see the other scripts that participated. I think especially History+ is a really cool idea and shows clever thinking and that you can actually do a lot more with UserJS than you might initially think.