I still wear T-shirts from vendors at tech conferences that I received years ago. They're durable, and they advertise the company not just to me but to others.
But perhaps the swag that has had the most staying power is stuff I could bring home for my kids to play with. I've gotten company-branded balls, puzzles, figurines, and other toys, and not only do they make a good "Guess what I brought home for you" surprise, but they sit around the house for ages, so you get a lot of exposure to their logos.
One thing I've never gotten as swag, but could see being a good ROI, would be a children's picture book, with a "Compliments of [Brand Name]" on it. With my kids, they would request that I read the same book to them 50+ times, so that's 50+ exposures to the brand.
Somewhat related to the children's book idea. I've received a Postgres coloring book from an Azure event.
Hopefully the PRs get upstreamed quickly without any opposition from Datadog unlike last time.
std disclaimer: I'm building oss obs for hyperdx, so I have a clear horse in the race for companies adopting open standards, just like grafana.