Engine Management Light (MIL) code reader?

Hi, does anyone recommend a diagnostic reader tool at a reasonable cost?  I suddenly have the MIL light on but no obvious cause on my 2004 WRX Wagon.

It follows a poor start from cold, after not being run for a few days.  Engine churned but eventually fired after several turns of the key, and spluttered a little before idling smoothly. I hope to sort the problem myself if I know what the code says.

Thanks for any advice