I have been using the navigation & real time traffic app Waze for a few years now, including some online editing of the maps.
Basically Waze is a community edited GPS navigation system that users can get live traffic from for free, the user's provide the data. Passively by driving with the app turned on, which continually reports speed and location dates back to the servers for analysis. This provides long term average road speed data and also can provide real time updates should the user be stuck in heavy traffic. Incidents can also be reported by Active reporting e.g. reporting stopped vehicles or other hazards for following drivers to watch out for, or even just point out errors in routing or the background maps.
As time has gone on the map data has improved tremendously, originally the UK maps were totally from user submissions and then a few years ago background map data was imported that filled in a lot of the gaps, it also imported some errors at the same time, but those have largely been ironed out thanks to the band of enthusiastic editors. Today the Waze UK maps are as reliable as most of the paid for GPS systems on the market, indeed they often get updates to new road layouts within days of the changes. Though there are, as with any other GPS some odd little errors now and then, but you can report these via a couple of taps on the screen and this will highlight the road to any map editors in the area.
The other app I use for traffic is the Inrix traffic App, which I started using last year, as well as a free version there is a paid version which allows a few extra features.
The Inrix app uses data from a variety of sources, not just the community, but road sensors and other sources. Inrix is consistently the only navigation app that I have found regularly has accurate road closures, the A34 dual carriageway has had regular slip road and carriageway closures overnight for the last year and the Inrix app has at least shown all the closures that have affected me.
Both apps have functionality to locate the cheapest fuel prices in an area, although Inrix again in my experience has more accurate info, Waze's facility to display and then navigate to a filling station along your route more than makes up for this.
I have found that the whilst Inrix app is excellent for planning and finding out traffic conditions prior to setting off, with the real time data being more comprehensive and accurate, it's lack of turn by turn navigation means that Waze is the app that I consistently have open when driving.