Subscription to telegram channels
Telegram has uber complex low level API (actually not an API but protocol — MTProto). I need to write Telegram client to read channels. Bot API doesn’t work for this, and querying channels one by one via client library quickly bumps into rate limit. So I pause this till Telegram makes normal API for getting contents of public channels.
Most Telegram channels have either RSS or VK/Facebook/Instagram equivalents to which you can subscribe.