Yesterday StanJames were giving out a £5 free bet just for emailing them ([email protected] just in case it's still working!), and that came out on Twitter.
Also, for a few more hours, The Gambling Times are giving away annual premium memberships just by following and retweeting, nly takes a minute and could be worth £49.99 to you!
https://twitter.com/TGT_Official
There's also a good article on which bookmakers to follow on twitter.