As people may recall, at the 2010 general election the good folk of Manchester Central awarded 153 votes to Socialist Labour, 54 to Socialist Equality and 59 to the Workers Revolutionary Party.
For the by-election they can share those numbers between the Communist League and the Socialist Party (of England and Wales, not of Great Britain).
... and according to a poster on Vote UK TUSC are likely to stand a candidate as well.
As I hinted on the Corby thread, the United People's Party website says they have disowned their previously announced candidate.
The Pirate candidate, Loz Kaye, beat the Lib Dems in Bradford ward in May's Manchester council election (and wasn't that far behind the Greens and the Tories).