JPEGmini. It's $20 for the full version, but you can trial it (as a test of quality) for free. The trial version (or, rather, the unactivated version) has a 20-image-per-day cap — no time bomb, though, so someone with different needs may never have to pay for it. The full version can recompress entire directories at a go.

It'll take most large high-quality JPEGS down to 20-25% of their file size (some more, some less) and you really need to do an image subtraction to see that it's made any change to the actual image at all. ("Large" means up to 28MP; if you've got a D800/D810/α7R or a medium-format camera and have left the JPEGs at your camera's full resolution, you'd have to spring for the "pro" version, and I'm not sure it's still cost-effective at the "pro" price.) It'll also do a batch resize at the same time if you want to do that.

It's slow as the proverbial molasses in January to load (it's a .NET application, and sort of feels like loading a full-featured Eclipse-based IDE), but it runs at an acceptable pace once it has finally loaded. (The "pro" version is considerably faster, but see the price issue mentioned above. Nothing's ever fast enough when you're doing a big job, but once the big job is over, you're left wondering why there's a Veyron in the driveway when all you use it for now is to go to the corner store for milk a couple of times a week.)

But do trial it on a few images before you buy — even cheap is expensive if it doesn't do what you want. The opinions of random weirdos on teh intarwebz ar often worth less than you paid for them. Oh, one more thing: the license fulfillment is done through B&H, so it's not instant. (Someone at B&H, lovingly known through the ages as "Kosher Kamera", has to press a button or something to make sure that you can't get it on Sabbath or a no-work-allowed holiday accidentally. Weird choice for a sole retailer of an electronically-delivered product in my opinion, but it's the choice they made.)

(No affiliation, just an otherwise satisfied customer who really wonders about the B&H part.)