I have a Pentax spot meter that I had calibrated by Quality Light Metric last year but I only use it perhaps 5% of the time as I generally use a pair of tiny Gossen Digisix meters, one lives in my LF pack permanently and the other is a floater from my Leica bag to my Hassy bag. The meter will fit in the smallest pocket you can imagine, battery life is outstanding and the long scale is really nice to have when extrapolating for things like ND filters. They read in EV numbers which over time are a lot easier to use as they commit to memory like EV 12 is 250th @F4, etc...

The spot meter app on the iPhone is also very good, works in a pinch every time.