I've used an eBay tach that has a Hall effect sensor, works pretty well as far as I can tell but I have no means to check its accuracy. Before I changed the motor, the speeds it gave pretty much matched the ones in my lathe manual, so it's at least in the right ball park. $12-13 plus a 9-15V wall wart to power it.
oh, there's also the Maxtach too - quite a bit more and a serious amount of self assembly required, but a lot more features (SFM/ tool diameter etc). Couple of people on here (Masteryoda?) have them and like them a great deal. I just couldn't spring for the extra