My mysql instance has 1700+ tables named "index_*". At 15MB each, this adds up to 25+ gigs.
How can I clean these up? Is it as easy as dropping these tables? or is there some configuration in tikiwiki that cleans up the database with regards to index tables?
Wow, 1700+ of them, never seen that before! Which version are you running? You probably need to upgrade as that sounds like a bug.
However, having said that the good news is that you can safely (but carefully) delete (drop) them and then rebuild the search index from the search admin panel or the command line using console.php and tiki will make a new one (or two).
I guess that 25 GB is too much to back up, but i'd suggest talking a backup of all the other tables if you can just in case.
The index_* tables are the unified search mysql engine's storage and it's usual to have a couple of them, maybe half a dozen or more, so something sounds like it's going badly wrong. Maybe you have a cron job running a regular rebuild? (but that would have to be every hour or something, usually once a day is plenty)
Good luck!