A little more generic solution (you can specify the number of parts you want, not just split 'in half'):

def split_list(alist, wanted_parts=1):
    length = len(alist)
    return [ alist[i*length // wanted_parts: (i+1)*length // wanted_parts] 
             for i in range(wanted_parts) ]

A = [0,1,2,3,4,5,6,7,8,9]

print split_list(A, wanted_parts=1)
print split_list(A, wanted_parts=2)
print split_list(A, wanted_parts=8)

good = [x for x in mylist if x in goodvals]
bad  = [x for x in mylist if x not in goodvals]

is there a more elegant way to do this?

That code is perfectly readable, and extremely clear!

# files looks like: [ ('file1.jpg', 33L, '.jpg'), ('file2.avi', 999L, '.avi'), ... ]
IMAGE_TYPES = ('.jpg','.jpeg','.gif','.bmp','.png')
images = [f for f in files if f[2].lower() in IMAGE_TYPES]
anims  = [f for f in files if f[2].lower() not in IMAGE_TYPES]

Again, this is fine!

There might be slight performance improvements using sets, but it's a trivial difference, and I find the list comprehension far easier to read, and you don't have to worry about the order being messed up, duplicates being removed as so on.

In fact, I may go another step "backward", and just use a simple for loop:

images, anims = [], []

for f in files:
    if f.lower() in IMAGE_TYPES:
        images.append(f)
    else:
        anims.append(f)

The a list-comprehension or using set() is fine until you need to add some other check or another bit of logic - say you want to remove all 0-byte jpeg's, you just add something like..

if f[1] == 0:
    continue

You can try something like that:

>>> items = ['foo bar', 'baz', 'bak foo bar']
>>> new_items = []
>>> for item in items:
...     new_items.extend(item.split())
...
>>> new_items
['foo', 'bar', 'baz', 'bak', 'foo', 'bar']