Positive online reviews on specialized websites. This is simlar to getting dugg or slashdotted, but the effect is far more lasting. It also helps with getting to the first page in Google & Co.
Sites, blogs, forums catering to the communities of users that you are after. Which ones these are exactly clearly depends on the nature of your product. For example, if it's a security-related application, it'd be nice to get a mentioning on Security Focus. If it's low-end networking stuff, something like tweakers.net might be a good fit. Etc ..
