Actually, Opera has already the ad blocker, named "content blocker", for some versions already. Find more details at this address: http://my.opera.com/Tamil/blog/ad-block
Opera has had ad blocking built in since at least 9.x, if not earlier. It does the job for me; I don't find it wanting. You can point and shoot to block anything that happens to get through the default filters, and the filtering takes wildcards by default, even. Meaning clicking an ad will block all ads from the same domain or parent path (e.g. adserver.tld/ads/*). Shift click to block just a specific image. The common text ads are also filtered.
I've used the same reason for avoiding Google Chrome. However, there is hope! I found an excellent "urlfilter.ini" list for Opera this morning.
so sad that this is the case, but it is :( adblock seems to have a record (for my personal use at least) as almost never having a false positive. privoxy on the other hand seems to block all sorts during actual use that aren't really ads, especially flash.
There's AdSweep at http://adsweep.org/ - I've never managed to get it to work on Opera 10 under Ubuntu, but I find it works well on Chrome and plenty of people have given it good reviews.