I work with a lot of charities, they often post volunteer opportunities on do-it.org but they will also offer opportunities on their own web pages too.
If you want to work with a charity they often have unadvertised volunteer vacancies (particularly for volunteer accountants as they find them difficult to source). If you want to work with a particular charity I would recommend picking the phone up and speaking to them. I'm a treasurer at two charities because I proactively asked them if they needed any help but then I do have substantial charity experience.
Beware though working with charities whilst very rewarding does has it's draw backs as you can get sucked in and it ends up taking over your entire life - you have been warned!!
I have been registered for a while with do-it.org but i cannot find any payroll related work. I applied for a finance assistant in Oxfard but i didn't hear from them :( . Its getting late now and am not getting a job in payroll :(