I'm sure there are some better ones out there but I went for one of these a while ago: http://cgi.ebay.co.uk/ELM-327-1-4a-Blue ... 513wt_1139
I can't complain for the money. Came with software and a bluetooth dongle for my laptop. Reads and resets error codes, does logging, displaying various parameters (although not all)
If you have an Android phone or Iphone with buetooth, you can download an app called 'Torque' (for free) and do all of the above from your phone aswell.