I'm not entirely sure this is a good fit for Stack Overflow... Just a quick question which hopefully many will find useful.
I see you can use hashtags, but that they have to correspond to existing channels. I have a channel tied to a project that gets a lot of automated notifications. I want to leave a message in that channel that I will later be able to find by searching through topics.
I do not want 100's of channels.
In contrast to twitter and other similar services in Slack the hashtag # is used to identify a specific channel, not for tagging a topic to a message. So #general is a reference to the channel names "general". As far as I know there is no mechanism in Slack to add tags to a message.
However you can achieve something similar with the search function. It will find any message that contains certain keywords. Use in: to narrow your search to a specific channel.