Open-Source Replacements for the SaaS Stack You're Bleeding On
PostHog, Cal.com, Plausible, Documenso, Mattermost, ERPNext — what to swap, what to keep, and what hosting actually costs.
PostHog, Cal.com, Plausible, Documenso, Mattermost, ERPNext — what to swap, what to keep, and what hosting actually costs.
Why open-source is back
The last two years produced a generation of open-source apps that look and behave like the SaaS products they replace. Self-hosting is not 2010 self-hosting anymore — most of these ship Docker images, ship managed-cloud editions, and have UX that does not embarrass anyone. The economics changed too: the SaaS pricing curve at scale has gotten steeper, while the cost of running a small Postgres-backed app has gotten cheaper.
Analytics: PostHog, Plausible, Umami
PostHog covers product analytics, session replay, feature flags, and experimentation in one tool — replaces Mixpanel + LaunchDarkly + Amplitude. Plausible and Umami cover lightweight web analytics. All three self-host on a tiny VM. Hosting cost: $30–80/month. Replaces SaaS bills routinely in the four to five figures monthly at scale.
Scheduling: Cal.com
Calendly equivalent. Self-hosted version is fully featured. We have shipped Cal.com behind several client domains for under $20/month each in hosting. Routing forms, team scheduling, conditional logic, calendar federation — all there.
E-signatures: Documenso
DocuSign / HelloSign equivalent. Open-source signing flow with audit trail. Not yet at full enterprise feature parity (no advanced workflow routing) but covers 80% of small-to-mid use cases. Hosting cost is negligible.
Comms + project: Mattermost, Plane
Mattermost replaces Slack for security-sensitive teams (audit trail control, on-prem option). Plane replaces Linear for teams with strong "we own our data" requirements. Both have come a long way; both are now legitimately good products, not "open-source as a downgrade."
ERP-ish: ERPNext
For sub-$50M companies looking to leave NetSuite, ERPNext covers the GL, inventory, manufacturing, and order-to-cash modules. Comes with Frappe framework underneath. We have shipped clients onto ERPNext + a few custom modules for 25–35% of their NetSuite spend, with the same workflow coverage.
When not to roll your own
Email deliverability (use SendGrid or Postmark — not your IP). Authentication for consumer apps (use Clerk or Auth0 until you outgrow them). PCI-scope payment processing (use Stripe). Statutory tax filings (Avalara). Anything where "we got it wrong" is a regulatory or business-ending event. Open-source is for the boring middle of your stack, not the regulated edges.