Dunno how into your cars you are, but Rev is flipping awesome. It's about £25 and you need a wireless OBD2 thingy which is another £100, but you can then set your iPhone up to read all the engine diagnostics and display them in a set of gauges. Currently have mine set up to read boost, air intake temp, mpg, engine load and speed. All on the one screen. That sort of set up using specialist gauges would normally cost about £500 and involve A LOT of tinkering under the bonnet. Rev took about 5 minutes.
Also, you can read, diagnose and clear any engine error codes. Considering you're looking at anywhere between £50-£100 a pop to get a dealer to do this for you, it's an absolute bargain.