Data, AI, and infrastructure workflows, orchestrated from declarative YAML: Kestra is an open-source, event-driven orchestration platform. Flows are declared in YAML - no DSL rewrites or Python decorators - and the definition stays the single source of truth even when edited through the UI, API, CI/CD, or Terraform, which makes pull-request review, versioning, and rollback natural. Tasks run in any language: Python, Node.js, Go, Rust, R, SQL, or Bash scripts executed in containers, and a plugin ecosystem of 1,000+ integrations covers ingestion, dbt, Airbyte, Spark, cloud storage, databases, and messaging systems. Scheduling supports cron triggers, event triggers, backfills, and conditional branching, with retries, timeouts, error handling, and typed inputs and outputs that surface artifacts in the UI. Namespaces, labels, and subflows organize workflows at scale, and the embedded code editor includes Git integration. Common uses span ETL/ELT pipelines, dbt runs, microservice coordination, infrastructure provisioning, and human-in-the-loop approvals. Java-based, Apache 2.0 licensed, deployed via Docker or Kubernetes.

Scheduling infrastructure, not just a booking page - Cal.com is the leading open-source scheduling platform. Share a link, attendees pick a slot, and real-time sync against Google Calendar, Outlook, and CalDAV prevents double-booking. Beyond the basics it covers team workflows: round-robin distribution, collective availability across multiple hosts, recurring meetings, and routing forms that ask bookers questions and send them to the right team member - the feature sales and support teams usually pay enterprise prices for. Paid bookings run through Stripe, video calls through the built-in Cal Video (Daily.co) or Zoom and Google Meet, and an app store connects 100+ tools including HubSpot, Zapier, and n8n. The API-first architecture with webhooks and embeds makes it practical to build scheduling into your own product, white-labeled with your domain and branding. Built on Next.js and Prisma over PostgreSQL, translated into 65+ languages, with the self-hostable community codebase maintained under an open-source license.

Buffer and Hypefury, taken on by an open-source, agentic social media scheduler: Postiz is a Next.js application with no feature gap between hosted and self-hosted versions. Connect 28+ platforms (X, LinkedIn, Instagram, TikTok, YouTube, Reddit, Facebook, Pinterest, Bluesky, Mastodon, Discord, Slack, and more), then draft, schedule, and analyze everything from a unified visual calendar. Platform-specific depth is real: Reddit flairs and subreddit search, YouTube playlists and categories, LinkedIn company pages and carousels, Pinterest boards, X reply controls. A built-in AI agent drafts hooks, captions, and threads tuned per platform, generates images from prompts, and can execute an instruction like "write a LinkedIn post about X, make a matching image, schedule it for Tuesday at 9am" end to end. The agentic angle extends outward: a CLI and MCP server let Claude, Codex, and other AI agents drive Postiz autonomously - discovering connected integrations, fetching platform constraints, uploading media, and batch- scheduling campaigns with structured JSON output. A public REST API plus n8n, Make.com, and Zapier integrations trigger posts from CI, a CMS, or any event source. Team features cover invites, comments, and collaborative scheduling. Self-hosting removes per-channel pricing and keeps OAuth tokens on your box.

Work, personal, business, and school calendars at different providers double-book because no one system sees your real availability - Keeper solves that multi-calendar collision problem. Its pull-compare-push sync engine aggregates events from Google Calendar, Outlook/Office 365, iCloud, FastMail, any CalDAV server, or read-only iCal/ICS feeds, and pushes blocking events to one or many destination calendars so time slots align everywhere. The design is deliberately content-agnostic - it syncs timeslots, not titles or descriptions, so a personal appointment shows as busy time on your work calendar without leaking details. Sync logic is clean: events Keeper creates carry a traceable UID suffix, deletions propagate, and orphaned entries are purged automatically. A token-authenticated aggregated iCal feed combines selected calendars into one subscribable URL for Apple Calendar or Thunderbird. An optional MCP server gives AI agents read-only calendar access over OAuth 2.1 - list calendars and query events by date range, with no write capability. Built with Next.js and Bun under AGPL-3.0, the standalone Docker image bundles web, API, cron, worker, Redis, and PostgreSQL in one container, and self-hosting unlocks every Pro feature - unlimited calendars and one-minute sync intervals - for free.

Service businesses get a booking page without per-booking commissions or monthly SaaS fees from Easy!Appointments, the self-hosted appointment scheduler. Customers pick a service, provider, and open time slot from a clean web form; the system enforces working plans, breaks, and booking rules you define per provider, then confirms by email to both sides. The structure fits real service organizations: multiple providers with individual schedules, multiple service types with their own durations and prices, and admin/secretary roles for front-desk management. Two-way Google Calendar synchronization keeps each provider's external calendar authoritative - book in Easy!Appointments and it appears in Google Calendar, block time in Google and the slot disappears from the booking form. Version 1.6 adds SMS notifications and payment support, and a REST API opens the scheduling data to custom integrations. Built on PHP (CodeIgniter) with MySQL, it installs in a single folder and can share a database with your existing site; a WordPress plugin embeds the booking form directly in pages or posts. The interface ships in dozens of languages with time-zone handling throughout. GPL-3.0 licensed and free for commercial use - a helpdesk, clinic, salon, or consultancy runs its whole booking workflow on its own server.

What Doodle did before ads and paywalls took over: Rallly (three L's) lets you propose a few dates, share a link, and watch an availability grid fill in - no email threads, no forced accounts, no "$6.95/month to remove ads." The availability grid makes the winning slot obvious at a glance, comments on each poll keep the "I can do Tuesday if we start late" discussion attached to the decision instead of buried in chat, and email notifications fire as votes and comments arrive. When consensus lands, finalize the winning option and everyone gets notified. The stack is modern TypeScript - Next.js, tRPC, Prisma over PostgreSQL, Tailwind - with a clean, genuinely mobile-friendly UI, dark mode, and community translations in 10+ languages. Self-hosting means unlimited polls and unlimited participants with meeting data on your server rather than a scheduling SaaS. It pairs naturally with Cal.com: Rallly answers "which time works for everyone?", Cal.com handles "book a slot on my calendar." AGPL-licensed.