Radicale
Calendar & Contacts
Simple calendar and contact server with extremely low administrative overhead.
Actively maintained
Real momentum
GPL-3.0
Every open-source calendar & contacts project we track, scored on maintenance, momentum and licence — so you can tell the live competitors from the abandoned repositories.
Calendar & Contacts has 8 open-source implementations in this dataset, of which 7 show real momentum and 0 are dormant or abandoned. The leader is Radicale with 4.9k stars against a category median of 1.2k — roughly 4x the typical project. Saturation score 40/100: Contested. Several maintained alternatives exist, but no one owns the category.
Winnable, but differentiation is not optional. Pick one underserved segment and be visibly better for it rather than competing feature-for-feature with a free tool.
The practical test in this category: Radicale is the reference implementation at 4.9k stars, and 7 of 8 projects are still shipping. A buyer evaluating you will find at least one maintained free option, so your pricing and positioning have to answer "why not self-host Radicale?" in the first minute.
Calendar & Contacts
Simple calendar and contact server with extremely low administrative overhead.
Calendar & Contacts
Lightweight CalDAV and CardDAV server based on sabre/dav.
Calendar & Contacts
Open source CardDAV, CalDAV, and WebDAV framework and server.
Calendar & Contacts
Calendar syncing tool that pulls and pushes events between calendar sources and destinations via iCal/ICS or OAuth, with support for anonymized busy/free events.
Calendar & Contacts
A simple, dockerizable and fully translatable admin interface for sabre/dav based on Symfony 5 and Bootstrap 4, largely inspired by Baïkal.
Calendar & Contacts
Manage my Damn Life (MMDL) is a self-hosted front end for managing your CalDAV tasks and calendars.
Calendar & Contacts
Open source CardDAV and CalDAV server with minimal administrative overhead, backed by a Git repository.
Calendar & Contacts
Server for calendar sharing (CalDAV) that uses a PostgreSQL database as a data store.
Existence is not competition. A project competes with you only if it is maintained, findable, deployable by your buyer and licensed for their use. Most projects fail at least one of those tests. Run all four before you shelve an idea — or before you assume you have a clear run.
Stars are a lifetime counter with no decay, so a project abandoned in 2022 still looks like a leader in 2026. In this dataset 149 projects are dormant, abandoned or archived, and several of them sit in the top decile by stars. Sort by momentum and commit activity instead — popularity tells you the project was once interesting, maintenance tells you whether it will still be there when your customer needs a bug fixed.
When a prospect says 'we could just self-host the free one', they are usually comparing your price against zero. The real comparison is your price against hosting, upgrades, backups, security patching, integration work and the engineer hours behind all of it. Quantifying that number is the single most effective response to a free-alternative objection — and it is more persuasive coming with a spreadsheet than with a feature list.
Founders treat licensing as a legal footnote and then discover it dictates strategy. Permissive licences allow closed commercial forks, which is why so many managed services exist around MIT and Apache projects — and also why your differentiation cannot be the code itself. Copyleft licences, especially AGPL, remove the closed-wrapper option entirely, which reduces competition but restricts you too. Source-available licences exist precisely to stop the wrapper business.
Search this database by category and by the commercial product you would replace. It covers 1665 projects across 130 categories, each mapped to the paid tool it substitutes. If your category shows a "Crowded" verdict, a maintained free version almost certainly exists and your positioning has to account for it.
No. It means free is your price anchor. Plenty of large companies were built next to a good open-source project — the ones that failed were the ones selling the same job at a price the free tool made indefensible. Find the part of the job the project deliberately does not do, and sell that.
Each category gets a 0–100 score built from three inputs: how many implementations exist, what share of them are still actively maintained, and how far ahead the leading project is compared with the category median. 62+ is Crowded, 38–61 Contested, under 38 Open. 11 categories are Crowded and 88 are Open.
A project has momentum when it committed code in most of the last twelve months, shipped a release recently and is not archived. 736 of 1665 projects qualify. This is the single most useful filter here — it strips out the popular-but-dead repositories that make a category look more competitive than it is.
It depends entirely on the licence. MIT, Apache and BSD let you build and sell closed products on top. GPL and AGPL require derivative work to stay open, and AGPL extends that to network use, which blocks most SaaS wrappers. BSL, SSPL and Commons Clause are source-available, not open source, and usually restrict competing commercial use outright.
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