Which coins use RandomX?

RandomX was designed for Monero, and Monero is still by far the best-known coin that runs it. But the algorithm is open and configurable, and other coins have taken it up, either as it is or as a variant of their own. The most practical list of them is the one inside the miner: XMRig keeps a table of the coins it knows by name, with the algorithm and the target time between blocks for each.
Coins on standard RandomX, rx/0
Monero (XMR). The coin RandomX was built for, on it since 30 November 2019, with a block every 120 seconds. Monero plans to move to RandomX v2 in a future hard fork — the RandomX v2 guide has where that stands.
Zephyr (ZEPH). Runs the same rx/0 as Monero, with the same 120-second block target in XMRig's table. Same algorithm does not mean same coin: Zephyr has its own chain, its own pools and its own addresses.
Townforge. Also rx/0, with a much shorter target of 30 seconds a block.
Coins on RandomX variants
Wownero (WOW) runs RandomWOW, rx/wow. ArQmA (ARQ) runs RandomARQ, rx/arq, the lightest variant. Graft (GRFT) runs RandomGRAFT, rx/graft. YadaCoin (YDA) runs RandomYada, rx/yada. In XMRig's table Wownero targets a block every 300 seconds and the other three every 120. The variants guide has what each variant changes.
XMRig also supports rx/sfx, the variant for Safex Cash, as an algorithm, though Safex has no entry in its table of coins.
Beyond the miner's table
The RandomX README names Arweave as one of the early adopters of RandomX; Arweave paid for the first of the four security audits in 2019. Outside both lists, the QRL project's improvement proposal QIP-009, of November 2019, set out a move of its mining to RandomX.
What a list like this does not tell you
A coin in a miner's table is a coin the miner knows how to talk to. It says nothing about whether that network is busy, how many people mine it, what it is worth, or whether a pool for it is still running — and coins come and go faster than miners' source code is tidied. Before pointing a machine at any of them, check that the chain is producing blocks and that the pool is paying. This site does not track prices or earnings, and will not.
Which coins use RandomX?: questions
Is Monero the only coin that uses RandomX?
No. XMRig's coin table lists Monero, Zephyr and Townforge on standard RandomX (rx/0), and Wownero, ArQmA, Graft and YadaCoin on variants of it. The RandomX README also names Arweave as an early adopter.
Can I mine Zephyr with my Monero settings?
With the same algorithm, yes — both run rx/0 — but not with the same pool or address. Zephyr is a separate chain, so the miner needs a Zephyr pool and a Zephyr wallet address.
Source: XMRig source code, src/base/crypto/Coin.cpp (xmrig/xmrig on GitHub); RandomX README checked against the source
More RandomX guides

P2Pool: mining RandomX without a pool operator
P2Pool is a decentralised Monero pool: a sidechain of its own, merge-mined with Monero, that pays each miner straight from the block, with no fee and nobody holding the funds. Its main, mini and nano chains, what it needs to run, and how a RandomX miner connects.

XMRig: the RandomX miner, set up and explained
XMRig is the open-source miner most RandomX mining runs on: CPU first, with OpenCL for AMD and a plugin for NVIDIA, and official builds for Windows, Linux, macOS and FreeBSD. Its RandomX algorithm names, the settings that matter, and the 1% donation.

What is RandomX? The CPU proof of work, explained
RandomX is the proof of work Monero has run since 30 November 2019: random programs executed in a virtual machine over a 2080 MiB dataset, built so that an ordinary CPU stays the efficient way to mine. How one hash is made, and why it needs that much memory.