A popular round-number port: OpenTSDB's metrics daemon and Quassel's IRC core both default here.
IANA has no individual entry for 4242 — it sits inside the 4200–4299 block registered as vrml-multi-use — so everything here is convention. OpenTSDB's time-series daemon leaves tsd.network.port required-but-empty in its source config and sets it to 4242 in the packaged one, and serves its HTTP API and metrics ingest there. Quassel's always-on IRC core also defaults to 4242 for its clients, and Code42's CrashPlan backup client used it as well. Fingerprint before assuming: an HTTP answer on /api/version is OpenTSDB, a binary handshake is more likely Quassel.
Expose with care
Which occupant it is decides everything. OpenTSDB has no built-in authentication at all, so a public tsd is an open read/write metrics store and should never face the internet; a Quassel core is designed for remote clients and is reasonable to expose, but only with TLS and a strong password.
$ curl -s --max-time 5 http://TARGET:4242/api/versionSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.