The Ready Reckoner provides in tabular form comparable freelance day rates for equivalent annual gross salaries for employees.
The Prince's Initiative for Mature Enterprise supports people over the age of 50 who are unemployed or facing redundancy to become self-employed.
The famous NUJ freelance fees guide. Required reading for freelance journalists.
I really like the 'Beermat' series of business books for start-ups. 'Marketing on a beermat' is particularly good.
Based on a book of the same name, which I recommend.
The NUJ, The Writers' Guild, Equity and the Musicians' Union have joined forces to help freelance members develop the key business skills needed to support career progress.
Notes from 2012 NUJ conference looking at new ways for freelances to make money.
Did you know the British Library has a Business & IP Centre? This wiki is one of their services.
US-based site for freelancers.