I came across these clip-on ND filters for GoPro Hero 3 recently and the results are interesting. The world's most popular action camera is also notorious for very high shutter speeds that result in side effects like "jelloing" where the edges of the footage seems to wobble. If you've ever used a GoPro for motorsports you'll know what I'm talking about. 

Adding a clip-on ND filter seems to slow down the hyperactive shutter and noticeably smooths out the footage. In addition you've got all the benefits of adding some Neutral Density filtering to your lens on a brightly lit day.

They're available for the naked/frame mount as well as the Hero 3 waterproof casing. So far I'm happy with mine although I suspect that they're not as tough as the GoPro casings themselves.

Available online from www.actiongear.co.za