Cluj-Napoca, Romania.
Per month:
Rent is 500-600 EUR max (worst case unless you're actively looking to splurge), with the average going around 250-300 or so.
You can cover food and taxes at a basic level on another 300. Of course, YMMV if you only order restaurant food and such, but if you stop by a supermarket every now and then you're set to go as low as 600 EUR/mth to live comfortably. I got by on 400 when I really had to in the past.
It's full of tech companies (most involved in outsourcing for Germany, the UK and the US) that pay decently for the appropriate experience - at least when you compare how much you'll have left at the end of the month vs UK and Germany for example.
There are several major universities around, foreigners are a common enough sight and virtually everybody (that you're likely to run into) born in the 90s or later speaks English on at least a basic level.
There are plenty of events on a general level, though if you're looking to find communities focused on very specific niches, you will likely have a harder time than in SF, mostly due to the smaller population size.
You can likely afford to not have a commute at all if you go in the 500+ range for rent.
I've been living in it ever since I started college in it many years ago, and weighing the pros and cons, I find it can really go toe to toe with a lot more famous cities in terms of the kind of lifestyle you can afford and the problems you put up with in exchange.
Not sure if US citizens require a visa or not.
LE: Based on points made in other posts - it's rated as one of the safest cities in Eastern Europe and has very affordable private health care. Getting a tooth fixed, for example, is around 40 EUR. A doctor's appointment is usually in the 25-40 EUR range for most specializations. Public healthcare, unfortunately, is worth avoiding if it can be helped.
Water has decent quality - you can drink the tap water - but a filter is not a bad investment. Air quality is rated quite high relative to most other cities in the EU as well.
Great net speed.