The only other usefull filters for digital photography are graduated ND filters. They help prevent overexposing the sky in landscape-shots, something that's hard to achieve for example in Photoshop.
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EOS 5D|EOS 600|15-30|24 1.4 L|135 2 L|2x 580EX|2x CP-E3|ST-E2|2x Pocket Wizzard Plus II|IXUS 850IS|Crumpler|Manfrotto|
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