I answered by email, but thought there might be someone else in the wild world of the internet that cares, so I thought I'd re-post it here.
The short answer is: not much. If you're not on the bleeding edge of technology trying to get every last gram out of your racing bike and willing to spend thousands of bucks to do that.
The single biggest thing, and the only thing I've actually bought (other than replacements for some gear that just wore out) is one simple idea: daytime running lights. The same high-capacity, rechargeable, lithium ion batteries that have pushed into our phones, tablets, laptops and so many other things have pushed into cycling headlights and taillights. Most of them are sold with a USB charger cable to plug into your laptop or other USB charger.
For my headlight, I bought a Cygolite Dash 460 headlight. These are more for being seen than seeing; at least from my use. With 460 lumens, though, they'll make a good headlight at night, too. Here's mine on my everyday ride - the weird background is a tire on my garden cart standing up in the shop.
Similarly, I got a Cygolite Hotshot Pro taillight, with 200 lumen output. In this case, though, I had originally bought a light recommended on a bike forum that I also got from Amazon, a Blitzu. This one gave me troubles and the USB charging port broke off the printed circuit board in the unit, leading to replacing it with this Cygolite. I would recommend the Cygolite over the Blitzu.
I have more experience with Cygolite as a brand. Back in the days when Mrs. Graybeard and I used to ride in the evening after work, I had a couple of Cygolite headlights and they still work (although the 10 year old NiCd and NiMH batteries have long since gone belly up).
Both my head and taillights are set to flash and they incorporate a pattern that isn't metronome steady. There are tests that say the irregular flashes are more noticeable and a regular flash. That said, I'm not aware of any studies that demonstrate riders with daytime running lights are safer than those without them, it just seems to be a reasonable bet. They should give me something like six hours on a charge, but I recharge them after each hour ride. I'll try long tests someday.
Let me just list throw out some of the other changes in the things I know
- Disk brakes and especially hydraulic disk brakes really took off. They're starting to make inroads into road racing, slowly, but the big growth has been for the two off-road style bikes: mountain bikes and:
- Gravel bikes. The last time I was riding a lot ('11 or '12? Before that?), the big thing was cyclocross - essentially road bikes with wider tires, and a more relaxed geometry. The idea was to race on dirt and gravel roads. Now the word appears to be gravel bikes for the same basic kinds of riding.
- Recumbent bikes and especially recumbent tricycles have continued to grow in popularity, although not as quickly as proponents have been saying since I started hearing it in about 1994. Recumbents are bikes that are ridden in a sitting position. Both two and three wheeled designs are popular; like everything else, they have good points and bad.
- The substitution of smartphones for all sorts of other devices on the bike. To the right of the headlight in my photo is a plastic case marked "CATEYE". That's my bike's speedometer and odometer. There are models on the market now that just have pickups near the back tire and display on the phone by Bluetooth. The same idea goes for heart rate monitors, GPS, route planners and any electronic gadget people might like to ride with.