If you liked Planet Earth, you should probably check out Nature's Great Events. Narrated by David Attenborough and currently airing in the UK on BBC1 and BBC HD, the series consists of six 50-minute shows, each of which features a large-scale annual event, like the spring thaw in the Arctic Circle and the sardine run along the coast of South Africa. The series was shot in HD using many of the techniques seen in Planet Earth.
If you're in the UK, you can check out the first three episodes on the BBC site. In the US, Discovery will be airing the show sometime in the spring under the title Seasons of Survival (apparently Nature's Great Events isn't dramatic enough for the American audience). No word on whether Attenborough's expert narration will also be replaced as it was in Planet Earth.
In the meantime, some HD clips of the show are available on YouTube. This slo-mo video of a grizzly bear shaking the water off its fur is fun to watch but this too-short clip of an extraordinary coordinated attack of dolphins, seals, sharks, and birds on a massive school of sardines is the gem.
(via we made this, who call the series "mind-blowingly good")