With the price of petrol in the UK and Ireland reaching record levels, some new Google Maps mashups will help you to find the station that has the cheapest:
MapEire: Cheap Petrol in Dublin - This mashup shows petrol prices for stations that have been submitted within the last 7 days. Stations are laid out on the map and when you click on them detailed information about the station pops up. You can change this information easily for future visitors to the map as well. MapEire contains information for Dublin carparks as well.
PetrolPrices.com (free reg. required) - This is a petrol price listing website that has integrated with Google Maps to show you the stations with cheap petrol around the UK. The site requires free registration in order to serve you relevant information about your location and the map even displays the icons showing you the station logo (BP, Shell, Murco, Total, Elf, Gulf etc...)
North American readers: Check out the "Cheap Gas" mashup.