I know some people on here use charity donation sites when doing sponsored walks etc. for charities.

When looking up such information I have come across three sites.

Home : Bmycharity - online fundraising for all

Virgin Money Giving | The best deal for charities | Helping raise more for charities

JustGiving for charities - JustGiving

All three have different paying arrangements and the first two are new onto the market compared with justgiving. I'd advise looking at all three and deciding then.