A few good options depending on your needs:

• Matrix (Synapse/Dendrite) — federated, E2EE, good client ecosystem (Element). Synapse is heavier on resources but full-featured. Dendrite is lighter.
• XMPP (Prosody/ejabberd) — lightweight, long track record, good for smaller deployments. Prosody is very easy to set up.
• Rocket.Chat — Slack-like UI, good for teams, but resource-hungry
• Mattermost — another Slack alternative, self-hosted, solid for team use

For personal/small group use, I would lean toward Prosody (XMPP) — super lightweight, runs on basically anything, and Conversations (Android) / Siskin (iOS) are decent clients.

For something more modern with a bigger ecosystem, Matrix with Dendrite is a good middle ground — lighter than Synapse and still compatible with all Matrix clients.