This is something that's been requested and worked on for a very long time, and should help a lot with the consistency of tags that people use on topics. It's also another significant feature that's been added by an open-source contributor: Shane Moore (whose Tildes username I actually don't even know) has been working on this on and off since last July, and has put up with me being slow to review and requesting some major changes to it over that time.
It applies to both the tagging field for new topics as well as the one for editing existing topics' tags, and the list of tags that show up for autocompletion are the 100 most commonly-used tags in each individual group (so the suggestions are different between ~tech and ~music and so on). This is just based on pure frequency at the moment (as in, the 100 tags that are on the most topics in that group), but in the future we could probably improve this to specifically include tags that have been getting used more recently, instead of looking at all time.
The interface can probably still use some work, and it's likely that there are some bugs and other issues with it, but as I've said before, Tildes is supposed to be in alpha! I haven't been adding nearly enough frustrating issues or breaking things, and we're all getting complacent with having a site where most things work!
Let me know what you think of it, and if you notice any issues. And thanks again for all the work and patience, Shane!