Kevin Kaufman.  He's a fabulous source of many good charts. =p

This thread on r21freak has pretty much every pack I've ever played.

The r21freak pack listing above is definitely the main source of content for the new Acme computer.  It's pretty much the authoritative pack list and they were nice enough to separate stamina packs from everything else.  There are a handful of packs with full difficulty sets and/or with ratings that look more like ITG 1-2 (that is, a few 12's and maybe a 13 or two, then mostly the 9-11 range). I've been thumbing through as I have time, and I'm generally happy with what I see - though there's still some crap, a lot of those packs have pretty well made charts and try to do interesting things without them feeling overly gimmicky.  There are a couple full-difficulty anime packs and one Touhou pack, none of which I have dug into too deeply, but which look good so far, and that'll make for an interesting Sakuracon this year.

If I'm looking for a specific song, Bemanistyle is the easiest place to search and have a few results come up.  While it's rare to find an amazing chart there, there will be a few charts for almost every song, and one of them will be good enough that it only needs 15 minutes of editing work to take out the couple stupid parts (and it'll be close to, if not perfectly synced, since people finally learned how to sync stuff, apparently).  Not to say you don't strike out sometimes and only find two charts which aren't even the right BPM, but in general, it gives you enough of a start that you don't just resort to writing your own.

Hahahaha thanks sara!

Everyone is right though. r21Freak has the most simfile packs on it that I can think of.

I always say give aaronin.jp a whirl because that's were a lot of ITG style packs are (a lot of them come from r21freak anyways) but they're thread has pretty much every major ITG tournament pack.
Btw if you're looking for groovestats. On AIJ they have all groovestat simfiles in one pack. It's something like 2GB of songs I think.

Thanks for the suggestions. I was looking more for individual songs instead of entire packs.

I do this too, but I usually don't modify the existing stepchart. Perhaps I should, because everything you said is too true. Usually the charts are SO CLOSE to being good, but have some stupid parts which just kill the entire chart.

For DDR files, I've acquired the highest quality charts from simfile tournaments (both ones I've ran and ones others have ran).

best-mix.net is the home where I've ran tournaments.
ddrevolved.com runs other tournaments.
Spork! has some good simfiles at ddrEncore.com

I recently discovered: http://divinelegy.com/

Many of the simfiles here are just great.  Start with the Jayce ones (which are available on some of the other sites mentioned in this post).

Does anyone have a link to where I can get TiK Tok or Down...the ones that are on the Acme Machine?