When it comes to an in-sink garbage disposal, I have little tolerance for unpleasantness. There's something totally unpalatable about smelling damp variety goo when pouring a glass of milk or peeling an apple nearby.
We used to be big fans of Plink, but then we found an elegant solution that is cheap, easy, and reuses kitchen waste.
According to my dad, one of the reasons the disposal smells is because little bits of nastiness get left behind. Apparently, if you run ice through your disposal, it knocks loose all the bits of cruft lurking down there in the dark. Dad also attests to the miracle power of ice to sharpen blades. I don't know if it's true, but in 11 years of home ownership where I periodically feed the disposal ice, I've not had disposal problems.
To get the super-fresh smell of lemons, we grind up frozen chunks of juiced lemon rinds with the ice. It's a brilliantly clean scent. Whenever we juice a lemon (limes work too!), we cut up the juiced rind into chunks no bigger than an ice cube and throw them in a container in the freezer.
Whenever the disposal gets monster breath, we throw in a cup or two of ice and a few pieces of rind (usually 1/3 of a lemon or 1/2 a lime). Run the disposal until the ice is gone then let the cold water run another minute.