In the past few weeks, both EasyJet and Ryanair have changed their policies. Read their web sites to see what it is.

EasyJet said on the web site that weight didn't matter so long as the item was within particular size limits. In my experience a couple weeks ago they seemed to be weighing carry-ons and limiting to 10 kg. Fortunately my carry-on was 10.0 kg and I didn't need to check any luggage.