The Woodland Trust has won the first Charities Online Accounts Award.
Winners in other categories were health education charity Life Education Centres, the Rett Syndrome Association UK, and Essex-based family support charity Special Needs and Parents.
The online awards, sponsored by the Charities Aid Foundation and the Institute of Chartered Accountants in England and Wales, replaced the previous print-based awards, which started in 1982. Only 86 charities entered this year, compared with 553 in 2002.
"The awards are designed to seek the best web-based annual report and accounts from across the non-profit sector, and aim to become a benchmark for good practice," said a CAF spokeswoman.
CAF chair Graham Ward, who led the judging, said the awards were based on a similar scheme for listed companies.
But he said that there was a surprising variation in the quality of online charity reports: "Some of the largest charities have excellent sites, easily on a par with anything in the commercial sector."