As of 2019-02, the Bitcoin Testnet Lightning Network has about 300 active nodes:
{% embed url="https://explorer.btc-test.ln.dead.cash/" %}
1ML indexes node and channel information for the Bitcoin Testnet:
{% embed url="https://1ml.com/testnet" %}
RADAR ION features a how-to guide for joining the Lightning Network, including a testnet bitcoin faucet, wallet and app recommendations, and a well-connected Lightning node to connect to:
{% embed url="https://ion.radar.tech/" %}
Several apps are live on the BTC Testnet:
{% embed url="https://starblocks.acinq.co/" %}
{% embed url="https://testnet.yalls.org/" %}
{% embed url="https://testnet.lightninggem.com/" %}
{% embed url="https://testnet.satoshis.place/" %}
{% embed url="https://example.coingate.com/" %}
{% embed url="https://testnet.wildln.com/" %}
{% embed url="https://spacebit.live/" %}