The list of all possible "goody hut" items is in the CIV4GoodyInfo.xml file in the XML\GameInfo folder. The only one that has any sort of requirement is the one that would heal a damaged ship, which requires the ship be damaged by 60% (or, possibly, to 60%). Some of them are for inhabited planets rather than wreckage, you can tell which is which by the RequiredImprovement tag.
The list of which goody you can actually get is dependent on difficulty level and is given in CIV4HandicapInfo.xml in the same folder. Each item in each list has an equal probability (except that scout type ships do not include the bad ones in the list) so if the list consisted of GOODY_HIGH_GOLD, GOODY_WARRIOR, and GOODY_TECH there would be a 1 in 3 change of getting each of the higher range of money, a destroyer class ship, or a tech. Adding a second GOODY_TECH item to the list would make that one have a 50% change and each of the other two a 25% chance.
Oh, and you can only get techs that are flagged as being available to get in the CIV4TechInfos.xml file. Currently, they are all set to be available. So if you manage to pop one in the late game you can get that very last not-yet-researched tech for free.
