Bonus cash promotions and welcome bonuses have always been a popular way for gambling sites to acquire new customers. But it's worthing remembering that nearly all these promotions will come with terms and conditions attached, make sure you do read them before joining.
Some offers like the one from 32 Red Casino do give you totally free cash, but most require you to make a deposit first before releasing bonuses. It's fairly standard industry practice to require you to wager/risk free bonus cash multiple times before you'll be able to withdraw it from your gaming account.
In my experience, you're often better choosing a site which rewards ongoing play, rather than those who just throw money at new players. Paddy Power sportsbook has great weekly offers for existing customers and Gala Casino regularly rewards players with ongoing bonuses and weekly specials.