Easyjet are generally very strict on the number of items you carry on board. Unless you've gone the "Easyjet Plus" route (via the card or the slightly more expensive seats which provide many other features, including dedicated check-in and faster boarding), you're allowed only one item with you on board. That includes your laptop, a handbag, or anything else. There's no separate count for bag and "accessory" as there is on some of the more traditional airlines.
Beyond that, there's no weight limit, as long as you are able to haul your bag into the overhead lockers yourself.
There are two different sizes for carry-on items: a smaller one is guaranteed (it basically needs to fit under the seat in front of you). The larger one is allowed, as long as there's space. If the overhead lockers are full and your bag is larger that the smaller size, they'll put it in the hold at boarding time. As far as I know, they won't charge you for this (if below the "larger" size).
Anything above the larger size is just a matter of luck (will they notice it?), which is not directly correlated with how full the flight is. If the flight is full, they're more inclined to check, but they may be busy and not notice it. If the flight isn't full, they're not motivated to check, but they have more time to do it...
Note that in many cases, it's actually a lot cheaper to add an extra checked bag rather than add weight to one. For instance, I just checked a Nice to Venice flight, and one 20 kg checked bag is 17 euros, while 23 kg goes up to 29 euros, and 26 kg to 41 euros! Obviously, you're better off selecting 2 bags of 20 kgs (which gives you a 40 kg allowance for 34 euros) rather than a single 26 kg bag!
Also note that if you have more than one bag on your booking (either because there's more than one person or you added more checked bags), only the total weight counts, not the weight of each individual bag (as long as no bag exceeds 32 kg -- that's for the health and safety of the baggage handlers).