Gas Buddy is what I use. Works very well. Paid for itself on the first fill up for me (40 gallon tank).
Not sure if everyone knows and/or realized this.....but many of these apps mentioned are actually just polling the Gas Buddy website. And, that only the stations that are reported by the Gas Buddy members will be shown. So, if no one has recently (usually within the last 72 hours) reported a price for that 'station closest to your house/job/current position', then a current price won't be shown on Gas Buddy's website, and hence it won't be shown on all the software apps that use Gas Buddy as a source for their info.
There is no single database that has every gas stations' current price. The sites (and apps) doing the reporting are getting the information from members that are providing the data real time as they purchase. I do this for several stations in my area, and those stations are usually the only ones that show up in my area.