I use both an X-Monitor and a Scan Gauge. They work well.
The Scan Gauge is the quickest / easiest to add, it just plugs into your OBDII (diagnostic) port. The X-Monitor is hard wired.
I've also reprogrammed my computer using the SCT LiveWire. Additionally, the LiveWire adds various gauges, similar to the ScanGauge but must also be plugged in to the same OBDII port. Since you can't have 2 things plugged in at the same time, I just use the ScanGauge, which I like better for that purpose. The LiveWire is a little big.
2005 Ford E-350 Diesel Sportsmobile
EB 102 Voyager top, 3.55 rear, Mobil 1 all around
270 amp alternator, billet water pump, coolant filter kit,
aluminum radiator, X-Monitor with pyro, trans temp, boost
+ Scan Gauge & SCT Live Wire (Economy)