I use a notepad app called Fast Notepad. It's free, but I'm not in love with it. I'm going to check out some of the others that have been mentioned here. But it's free, so it's worth a try if you don't have a notepad app that you like.
I bought Shop it, and I like it. It seems like it's much better for timing stuff than for notes, but I've only had it for a couple of weeks and haven't played with the note function that much yet. It makes timings easier, but I don't do a ton of shops where multiple timings are needed. Before I got Shop it, I used the regular stopwatch feature on the Clock App on my Samsung. I find Shop it a bit easier and more stealthy for timings than the stopwatch was, but unless you have a shop with lots of timings, I'd consider Shop It as a nice-to-have, not a need-to-have.
Shopper in California's Bay Area