Is there a free online chess repertoire tool? It can be some place where you register, save variations and comments, and navigate with a board what you do. I was used to chessply, but apparently it disappeared.
In addition to the other answers, there are a few sites to consider. These include annotation and comment features as mentioned by the OP.
Upload a PGN and then you play the computer interactively. You play moves from your repertoire and the computer plays the opponent moves. This is also open source.
Similar functionality in the "Opening Trainer." The interface is a bit different but it tracks what lines you have learned a bit differently which is nice.
As B.Swan has pointed out in a comment, the Study feature includes this. Further, Hauptideal points out in a comment you can use the Analysis feature's book icon to access master statistics, lichess statistics, and statistics of any lichess account you want.
There are also other related sites worth exploring to understand their capabilities and how they may supplement the resources above.
Import/export PGNs. Search databases, get statistics. See opening relationships in a graphical tree.
Import/export PGNs. Import and analyze a user's openings from a database, PGN, etc, including popular online chess sites.
There is already an offline program which has a lot of my ideas implemented:
But there is no good online tool which comes even close to that, let alone surpassing the linked offline tool. And I think an online tool, which stores the individual repertoire databases on a serve, would be much better, because then you could use the tool from any device, PC, tablet or smartphone. And if professionally made the online tool could easily surpass the offline tool linked above.
This opening explorer is so incredibly comprehensive and intuitive that I cannot imagine anything better. Just make sure you use the website, not the mobile app. The app is trash. The website is better optimized for mobile than the mobile app.
I was looking for the exact same thing and could not find what I wanted so I ended up building this free tool mychessnotebook.com ! It is a text editor which will automatically replace FEN formulas with pictures representing the chessboard (upon PDF export).