Bloggers like to give and win prizes. It offers some excitement for readers, content and links for the blogger, and if the prize is offered by a third party it gives them some exposure too. Darren does this a lot and to good effect, I do it very occasionally when I have something good to provide as a prize.
One question I get asked is how exactly do I select a random comment? You probably can make or find a plugin but I just use some simple SQL to read the WordPress database.
Most hosting providers offer a PHPMyAdmin tool so you can run the SQL through a simple web interface. All you need then is your post ID which can be found in your WordPress post editing screen. Hover over the Edit link for the post you want to select comments from and it will say in the status bar a URL ending in ‘post=’. The number that follows is what you need to note down.
In my example the post was numbered 356 so my query ends up as:
WHERE `comment_post_ID` = 356
ORDER BY RAND()
LIMIT 3 ;
Rand does the job of making the results random. Limit says how many results you want, if you only have one winner then make it 1.
As I said, no doubt someone has made or will make a plugin, but being a geek I find this quite efficient enough!