This Thai sauce will always be found next to street side hawker grilled chicken in its homeland. It is fairly easy to find bottled in Asian markets, but to my palate it is way too laden with vegetable gum, an artificial thickening agent, making it gluggy and unappetizing. Instead, try making your own.
As ole simonc says, it is a pinch to make at home and is deliciously addictive not only with the aforementioned chicken, but with any grilled dish (seafood, tofu, or spring rolls).
Here is a slight variation on the recipe above, it results in a thinner sauce than the bottles, and all the tastier for it.
Finely chop the first three ingredients, or pound in a mortar and pestle. Combine with the vinegar and sugar in a saucepan and simmer for 15 minutes. Add a small splash of fish sauce to season. Let cool, then serve.
The coconut vinegar is the secret ingredient, it has a mellow, tropical sourness. If you have trouble finding some, don't simply substitute white vinegar, it is way too harsh and acidic. Try 125 ml white vinegar mixed with 125 ml (½ cup) water and a squeeze of lime juice.