All rclone backends:

  alias        Alias for an existing remote
  acd          Amazon Drive
  azureblob    Microsoft Azure Blob Storage
  b2           Backblaze B2
  box          Box
  crypt        Encrypt/Decrypt a remote
  cache        Cache a remote
  chunker      Transparently chunk/split large files
  compress     Compress a remote
  drive        Google Drive
  dropbox      Dropbox
  fichier      1Fichier
  filefabric   Enterprise File Fabric
  ftp          FTP Connection
  gcs          Google Cloud Storage (this is not Google Drive)
  gphotos      Google Photos
  hdfs         Hadoop distributed file system
  http         http Connection
  swift        OpenStack Swift (Rackspace Cloud Files, Memset Memstore, OVH)
  hubic        Hubic
  jottacloud   Jottacloud
  koofr        Koofr
  local        Local Disk
  mailru       Mail.ru Cloud
  mega         Mega
  memory       In memory object storage system.
  onedrive     Microsoft OneDrive
  opendrive    OpenDrive
  pcloud       Pcloud
  premiumizeme premiumize.me
  putio        Put.io
  qingstor     QingCloud Object Storage
  s3           Amazon S3 Compliant Storage Providers including AWS, Alibaba, Ceph, Digital Ocean, Dreamhost, IBM COS, Minio, SeaweedFS, and Tencent COS
  seafile      seafile
  sftp         SSH/SFTP Connection
  sharefile    Citrix Sharefile
  sugarsync    Sugarsync
  tardigrade   Tardigrade Decentralized Cloud Storage
  union        Union merges the contents of several upstream fs
  uptobox      Uptobox
  webdav       Webdav
  yandex       Yandex Disk
  zoho         Zoho

To see more info about a particular backend use:
  rclone help backend <name>