Build your own website! This website is made on Google Sites, and the process was pretty intuitive. It's also free, but has no bi-directional links.
This is a free notetaking app (with paid options), that's attractive, relatively easy to use and has bi-directional links called 'backlinks'. Here's an example of a French digital garden built using Notion: Dans Les Bois (https://philipb.cc/) .
I haven't tried this, another free notetaking app with bi-directional links, but here's an example of a digital garden built using Obsidian: Hermitage (https://hermitage.utsob.me/)
A more visual and hip way of collecting ideas and short pieces, with bi-directional links called 'connections'. It's not so good for writing extended pieces, so it's less text based than many others on this list. It has free and paid subscriber options.
Click here for an example of an Are.na digital garden about... digital gardens (Hoff, 2022)!
Here's the link to the Are.na website (https://www.are.na/).
Wordpress isn't as 'indie' or new as the other options, but still a good free alternative. Chuck Grimmett has pointed out that the old-fashioned blog 'pingback' can operate as a bi-directional link (Grimmett, 2020).
Here is a link to the digital garden made using a customised WordPress site: Chuck's Notes (https://notes.cagrimmett.com/).
Read the following articles (Appleton, ca. 2021; Le Cunff, 2020c) for more details.