The old brain isn't working so thought I'd share this list I've compiled, ordered by numbers of followers.
National Union of Students - 1,675
Workers Uniting - 1,317
Equity - 1,145
Unite - 1,060
Unison - 904
NUT Students - 324
Broadcasting Entertainment Cinematograph and Theatre Union - 301
Public and Commercial Services - 290
University and College Union - 275
Trades Union Congress - 216
I had to struggle to find these. It seems large swathes of the trade union movement are yet to be won round to the benefits of Twitter. Especially surprising is the absence of the National Union of Journalists and the Communication Workers' Union!
At least the Socialist Party has climbed aboard the good ship Twitter. You can follow Marxist tweets for labour and youth here.