Web browsers
Text message replacements (cross-platform, secure)
Direct file transfer (secure, no cloud)
OnionShare, Datash, SendSecure, JustBeamIt
File Transfer (secure, temporary cloud)
Tresorit Send, WeTransfer, UploadFiles, FileMail
File sync (no cloud)
Password management (no cloud)
KeePassXC (hint: combine with SyncThing)
Full Cloud
(e.g. google/icloud alternative: email, contacts, calendar, files, etc.)
File Sharing, Chat, Team Collaboration
(e.g. Slack alternative: private, secure, with cloud)
Secure Email
VPN (Virtual Private Network)
ProtonVPN (free & paid), NordVPN (paid & good)
Free Office Suite (Microsoft Office Alternative)
LibreOffice (better MS compatibility), OpenOffice
More free and open source source software
14 Best Free And Open Source Software For Windows 2018
11 Best Open-Source Software 2019