General Thread Taps and Dies
HarryMann: A good set of taps and dies can get you out of an awful lot of trouble - at critical spanner throwing moments!
This one is Silverline's "Expert" set, up to M12. About £30. (Currently Dec 2009, Amazon.com doing these for £26-£29) I think that is good value, the tap wrench is crap, work for a year or two, but the taps themselves are very good IMO. One tends to use dies less often, but again they are HSS
Didn't expect it to be much cop actually, but has proved with care to cut threads in semi-nimonic cast steel exhaust manifold, s/s and whatever I've had to use it for really. The tap wrench and die holder aren't a lot of cop, but the taps and dies are def. OK for general use, though not actually the best (which can cost £30 for just one!) All 2nd cut taps, no bottom taps or first taper, but can always buy the odd bottom tap if really needed.
I wouldn't buy their 'non-expert' set though :)
See also Threads, metric conversions etc. Get a good thread file too...
P.S. If you haven't tapped threads before, try to learn a few tricks - e.g. one forward, 1/3 back, repeat... backing it out regularly and cleaning, don't hit bottom with a tap, cutting oil, and always take great care when starting.