Look at the box that your chain came in -- 6/7/8 speed chains will either be 7.1 mm or 7.3 mm pin length.
The SRAM chains are 7.1 and Shimano are 7.3. KMC notes this on their website. Get the one that matches your chain brand (and if its KMC, get the one KMC suggests). Wipperman's lineup seems to be a mess -- some of the 8 speed are 6.6, others are 7.2; it might just be an error on their website though.
The 7.3 mm missing link is known as the "KMC MissingLink I" and the 7.1 mm one is the "KMC MissingLink II". The 9's are 6.6 mm and the 10's are 5.9mm (which may be useful for someone who is confused with a higher speed chain, or another manufacturer's chain).
In practice, a lot of people use another manufacturer's quick link (KMC/Wipperman/SRAM) with another manufacturer's chain. I've used SRAM 8 speed Powerlinks in Shimano chains without problem. However, official manufacturer guidelines is generally to use their link closing method -- Shimano, use their special rivet, SRAM use the provided Powerlink/lock(for >9 speed), KMC is usually links, etc. But, normally, you get a link with the chain, and carry whatever link for emergencies; any of the links on the market for the right number of speeds will do for emergency repairs.