Here are some alternatives:
Nextcloud is the best Dropbox replacement due to its feature-rich platform, strong community support, and extensive customization options. It closely mimics Dropbox’s functionality.
Syncthing is ideal if you want a lightweight, no-cloud solution for syncing files across devices.
OwnCloud or Seafile are great for team collaboration, with ownCloud offering more compliance features and Seafile focusing on speed.
SparkleShare is a niche but effective choice for projects requiring Git-based versioning.