There Is A Bot For That is described as 'Bot discovery platform to help find useful bots'. There are eight alternatives to There Is A Bot For That, not only websites but also apps for Slack. The best alternative is Slack App Directory, which is free. Other great sites and apps similar to There Is A Bot For That are BotArena (Free), BotPages (Free), Botlist (Paid) and Bot Stash (Free).
BotsBook.com is a catalog of useful bots for main messenger platforms such as slack, skype, telegram and facebook messenger. It's like imdb - but for bots... You may add a bot absolutly for free to this catalog.