VRChat Tracker is an unofficial companion / assistant / helper app that lets you manage your VRChat friends, invites and avatars in a convenient way, with multiple sorting options. The app was released in early 2019 and is still gaining features.

Some of the app's features:
Backup your favorite worlds and avatars.
Add notes to your friend profiles.
Get notified when chosen friends are online.
Look up the author of the avatar that a user is currently using.
Browse missed invites.
Check hidden user ranks.
Search for worlds and users.
View your recent worlds.
Manage your avatar list.
Switch your avatar's privacy to public or private without reuploading.
Change your status description and biography.
A lot more features planned.

Please note that you cannot login with a standalone Steam or Oculus account but you can easily merge it into proper VRChat account by following the official VRChat guide https://help.vrchat.com/hc/en-us/articles/360062659053-I-want-to-turn-my-Steam-Oculus-or-Viveport-account-into-a-VRChat-account

There is an option to support the development by purchasing dark theme within the app.
Read/write external SD data permission will be prompted if you decide you want to backup your app data into a file so that it does not get lost.

In case you were wondering about viewing and favoriting public avatars - it was removed on VRChat's side from the VRChat's API which the app uses. It doesn't look like it's coming back.

If you have any questions, requests or encountered bugs, feel free to email me at vrctracker@gmail.com.