That link Fubar shared has ~4,700 maps on it. If you were to estimate 10 Megabytes per map, that's around 45 GB. That's probably not even a comprehensive list of every map, either (and there isn't any mass download feature either).
To my knowledge, there aren't any good packs of maps that you can just download all at once. There is the
Swamp Map Pack, but it's only 4 maps.
If you want some pointers on where you can grab some tested-and-proven-fun maps, check out our
Map Showcase.
This thread also has a few lists that people compiled of maps they'd like to see go official.