I've always had a soft spot for the Countdown numbers game, a challenge pitched just perfectly right for my mental capacities. It would take a very good numbers game to displace Countdown in my heart.
Mathador is a very good numbers game1 with similarities to Countdown: you're given a batch of numbers, a target, and a countdown timer. The differences, though, are where it's interesting: instead of getting to your target in 30 seconds, you need to get as many correct sums in three minutes as you can; using different operations earns you different numbers of points; and you get a hefty bonus for completing a 'Mathador' -- using all four operations in reaching your target.
The targets are lower than in Countdown (generally below 100) and, so far as I can tell, always solvable with the five numbers you're given. There's a social aspect to it (you can play head to head, and there are leaderboards for the day, week and month) as well as the now-ubiquitous gamification badges.
There's not much to dislike about Mathador -- I'd like a keyboard play option (I think my speed would improve dramatically) and I'd like a version that fit nicely on my laptop screen (honestly, it's vector graphics, how hard can it be to resize things nicely?). I've yet to download the app, for fear of never surfacing from my phone.
For most, though, I fear the biggest obstacle to enjoying Mathador is that the instructions and menus are in French; while that's fine if you speak decent French (I do) or can use Google Translate (you can), it's still not clear exactly what some of the rules are and what some of the badges mean.
That said, it's an excellent diversion, the kind of thing that makes an excellent short end-of-task treat for the arch-procrastinator, and I recommend it heartily.