ProGold ProLink Chain Lube
We use this in the shop and on our own bikes. It's a thin lube, so it needs to be applied more frequently (especially if it rains), but it won't pick up sand and grit like other thick lubes.
Regularly lubing your chain saves you money in the long run, because it means you don't need to replace your chain as often. Remember to use sparingly (one drop per link), and afterwards wipe down the chain to remove as much excess lube as possible.
Comes in an easy-applicator 118mL bottle.