There are hundreds of pounds worth of free bets available with bookmakers just for opening new accounts and placing qualifying bets.
The usual way is that the bookmaker will match your opening bet with a free bet of the same value, up to a maximum amount (ie, £50).
I would recommend that before placing any bets, it is wise to check the terms and condidtions of all the individual bookmakers as most have rules relating to what bets are, or are not acceptable.
For example, some bookies will only allow bets on odds of better than evens for a qualifying bet, or free bet. Thats why it is important to know what the rules are before parting with any cash.
These free bets can also be used for Matched Betting.
888Sport - Free £20 bet if your opening bet loses.
Bet365 - £200 in Free Bets
Betfred
Coral
Sportingbet £50
Stan James
VC Bet
Paddy Power