The best thing is the 4 duvet days a year on top of your annual holiday allowance for those days (mainly hungover ones) where you just don't feel like coming into work. The commission structure is great, with an accelerator for billing more, which is even more of a motivation to work hard! The quarterly incentives are always well thought-out and a lot of fun.
Holiday incentives, Regular End of Quarter Rewards, Duvet Days (4 a year, for when you just don't feel like going to work today, whatever the reason), Friday Drinks in the office, regular visits to the pub.
One of the best things is Duvet Days. You get 4 a year, and simply call up in the morning say the words duvet day and no questions will be ask, it's your day off to do as you choose. Aside from that we have beers at work every Friday, early leave, birthday drinks, regular outing and events, as well as the awesome company holiday twice a year.
List based on reports from current and former employees. It may not be complete.