Yes, I know they pay for sites, but ads are just too in-your-face, irritating and dumb. Not on all sites, but blame the sites that have flashing ads, video ads, ads for junk, even ads with sound. Here's how to block ads in Opera.
Go to Tools>Advanced>Blocked Content. Now all you have to do is enter some of the most common ad server web addresses. Addresses have to be entered one by one. Blogote has a short list that doesn't take too long to enter and seems to be effective.
Opera stores the web addresses in a file called urlfilter.ini. A longer list of ad server addresses can be entered into this file. Tamil has A full description and a list of ad server addresses.
urlfilter.ini does not exist by default. Use the first method to add a few addresses and Opera will create the file. The longer list can then be cut and paste in, making sure addresses follow the same format.
Update: Opera 11 now has extensions- and there are a couple of ad blockers available, so this post is now redundant.