i work for a vehicle tracking company. having recently developed a new mapping search tool for our system, i found it impossibly hard to locate a reliable webservice or source to get postcode data from. royal mail want something like £3k/year, which gives you updates once every quarter or something. which is quite a lot really for what is basically a csv file about 10 mb's in size.
i've ended up attempting to compile my own, with a mashup i developed by using mappoint (to which we are a subscriber to) for full length postal codes, but for shorter sector-based codes that royal mail's database will define, mappoint doesn't. but geonames.org does (well, for the first sector it does). but that would mean i'd have to find another source of data for second sector codes. it's ridiculous! it makes it seemingly impossible, without having to pay through the nose, for a reliable service to get this data from one location, that is going to be complete.
i fully stand by these open source/open development projects. these are what this internet (and country) needs. a location to build a reliable database of what some monopolizing arseholes want to charge you extortionate money for. good on you, guys!