The simple tool I wrote that I get the most bang for my buck out of is synesthesia[1]. I spend a lot of my time tracing things down across multiple log files, and having to pick out the important lines visually or trying to isolate them with custom grep incantations was wearing on me. Synesthesia allows you to specify regexes, and it will color matches based on the value of the match itself, meaning that it's stateless and doesn't need to keep a dictionary of strings to colors. This makes keeping track of things like GUIDs easy - you can just keep track of e.g. the orange one and watch it fly by across multiple terminals. It's currently python 2 only and assumes a 256 color terminal, but it has been invaluable.

I've been toying with using the idea for forums so that it is easier to keep track of who is replying to whom[2]. I also would like to try using it as a layer on top of traditional syntax highlighting, perhaps as an emacs minor mode - if those can provide colors to the buffer; I've written hardly any elisp and don't know what capabilities are available.

[1] https://github.com/cromo/synesthesia [2] https://imgur.com/E1N1Zsm