Slack's integrated upsell prompts

Slack offers an online workspace where teams can chat and send messages about projects and updates. Working on a freemium model, Slack entices free users to upgrade to the paid version with well-timed in-app messages.

Why this is really good UX:

• Slack shows users the first upsell message once they reach the searchable 10K message limit. Any user who reaches this point has definitely experienced the product's value.
• Slack integrates similar upsell prompts throughout the app when users take relevant actions like searching through their history or reviewing a conversation.
• An upsell message appears also in the user settings with a neat chart that visualizes current usage on the free plan.

You can read more about Slack's upsell prompts and their effectiveness here.