SethBling's BanSlimes filter, with two of my fixes.

BanSlimes.fix1.py

# Feel free to modify and use this filter however you wish. If you do,
# please give credit to SethBling.
# http://youtube.com/SethBling

#from pymclevel import TAG_Long

# Performs signed overflow and underflow for Java's long.
# NOTE apparently not required.
def jlong(x):
	# Signed overflow.
	if x >= (1 << 64):
		return -(1 << 63) + (x & ((1 << 64)-1))
	# Signed underflow.
	if x < -(1 << 64):
		return (1 << 63) - 1 + (-x-1 & ((1 << 64)-1))
	return x

# This mimics some of the functionality from the Java Random class.
# Java Random source code can be found here: http://developer.classpath.org/doc/java/util/Random-source.html
# NOTE not used directly, inlined below.
class Random:
	def __init__(self, randseed):
		self.setSeed(randseed)

	def setSeed(self, randseed):
		self.randseed = (randseed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
		
	def next(self, bits):
		self.randseed = long(self.randseed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)
		return int(self.randseed >> (48 - bits))

	def nextInt(self, n):
		while True:
			bits = self.next(31)
			val = bits % n
			if int(bits - val + (n-1)) >= 0:
				break
			
		return val

# Algorithm found here: http://www.minecraftforum.net/topic/397835-find-slime-spawning-chunks-125/
def goodSeed(minx, minz, maxx, maxz, seed):
	for x in xrange(minx, maxx):
		# Partial seed for the current x.
		partial = seed + (x * x * 0x4c1906) + (x * 0x5ac0db)
		for z in xrange(minz, maxz):
			# Compute the seed passed to the Java Random class.
			randSeed = (partial + (z * z * 0x4307a7) + (z * 0x5f24f)) ^ 0x3ad8025f
			
			# Random.setSeed(randSeed)
			randSeed = (randSeed ^ 0x5DEECE66D) & ((1 << 48) - 1)
			
			# Random.nextInt(10)
			while True:
				# bits = Random.next(31)
				randSeed = (randSeed * 0x5DEECE66D + 0xB) & ((1 << 48) - 1)
				bits = randSeed >> 17 # 17 == (48 - bits) with bits == 31
				
				val = bits % 10
				if (bits - val + 9) >= 0: # 9 == (n-1) with n == 10
					break
			
			slimeChunk = val == 0
			if slimeChunk:
				return False
	
	return True

inputs = (
	("Max Seed", 100000),
)

def perform(level, box, options):
	# Convert block coords to chunk coords
	minx = int(box.minx/16)
	minz = int(box.minz/16)
	maxx = int((box.maxx-1)/16)+1
	maxz = int((box.maxz-1)/16)+1
	
	for seed in xrange(options["Max Seed"]):
		if goodSeed(minx, minz, maxx, maxz, seed):
			#level.root_tag["Data"]["RandomSeed"] = TAG_Long(seed)
			print "Found good seed: " + str(seed)
			return
	
	print "Didn't find good seed."

# Code for testing the speed of the brute-force.
class TestBox:
	minx = 0
	minz = 0
	maxx = 10*16
	maxz = 11*16
perform(False, TestBox(), {'Max Seed': inputs[0][1]})

BanSlimes.fix2.py

# Feel free to modify and use this filter however you wish. If you do,
# please give credit to SethBling.
# http://youtube.com/SethBling

#from pymclevel import TAG_Long

# Performs signed overflow and underflow for Java's long.
# NOTE apparently not required.
def jlong(x):
	# Signed overflow.
	if x >= (1 << 64):
		return -(1 << 63) + (x & ((1 << 64)-1))
	# Signed underflow.
	if x < -(1 << 64):
		return (1 << 63) - 1 + (-x-1 & ((1 << 64)-1))
	return x

# This mimics some of the functionality from the Java Random class.
# Java Random source code can be found here: http://developer.classpath.org/doc/java/util/Random-source.html
# NOTE not used directly, inlined below.
class Random:
	def __init__(self, randseed):
		self.setSeed(randseed)

	def setSeed(self, randseed):
		self.randseed = (randseed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
		
	def next(self, bits):
		self.randseed = long(self.randseed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)
		return int(self.randseed >> (48 - bits))

	def nextInt(self, n):
		while True:
			bits = self.next(31)
			val = bits % n
			if int(bits - val + (n-1)) >= 0:
				break
			
		return val

# Algorithm found here: http://www.minecraftforum.net/topic/397835-find-slime-spawning-chunks-125/
def initChunks(chunks, minx, minz, maxx, maxz):
	for x in xrange(minx, maxx):
		# Partial seed for the current x.
		partial = (x * x * 0x4c1906) + (x * 0x5ac0db)
		for z in xrange(minz, maxz):
			# Compute per-chunk position-dependent partial seeds.
			chunks.append(partial + (z * z * 0x4307a7) + (z * 0x5f24f))
	return chunks

def goodSeed(chunks, seed):
	for i in chunks:
		# Compute the seed passed to the Java Random class.
		randSeed = (i + seed) ^ 0x3ad8025f
		
		# Random.setSeed(randSeed)
		randSeed = (randSeed ^ 0x5DEECE66D) & ((1 << 48) - 1)
		
		# Random.nextInt(10)
		while True:
			# bits = Random.next(31)
			randSeed = (randSeed * 0x5DEECE66D + 0xB) & ((1 << 48) - 1)
			bits = randSeed >> 17 # 17 == (48 - bits) with bits == 31
			
			val = bits % 10
			if (bits - val + 9) >= 0: # 9 == (n-1) with n == 10
				break
		
		slimeChunk = val == 0
		if slimeChunk:
			return False
	
	return True

inputs = (
	("Max Seed", 100000),
)

def perform(level, box, options):
	# Convert block coords to chunk coords
	minx = int(box.minx/16)
	minz = int(box.minz/16)
	maxx = int((box.maxx-1)/16)+1
	maxz = int((box.maxz-1)/16)+1
	
	# Initialize per-chunk seeds.
	chunks = initChunks([], minx, minz, maxx, maxz)
	
	for seed in xrange(options["Max Seed"]):
		if goodSeed(chunks, seed):
			#level.root_tag["Data"]["RandomSeed"] = TAG_Long(seed)
			print "Found good seed: " + str(seed)
			return
	
	print "Didn't find good seed."

# Code for testing the speed of the brute-force.
class TestBox:
	minx = 0
	minz = 0
	maxx = 10*16
	maxz = 11*16
perform(False, TestBox(), {'Max Seed': inputs[0][1]})

BanSlimes.original.py

# Feel free to modify and use this filter however you wish. If you do,
# please give credit to SethBling.
# http://youtube.com/SethBling

#from pymclevel import TAG_Long

# This mimics some of the functionality from the Java Random class.
# Java Random source code can be found here: http://developer.classpath.org/doc/java/util/Random-source.html
class Random:
	def __init__(self, randseed):
		self.setSeed(randseed)

	def setSeed(self, randseed):
		self.randseed = (randseed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
		
	def next(self, bits):
		self.randseed = long(self.randseed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)
		return int(self.randseed >> (48 - bits))

	def nextInt(self, n):
		while True:
			bits = self.next(31)
			val = bits % n
			if int(bits - val + (n-1)) >= 0:
				break
			
		return val

# Algorithm found here: http://www.minecraftforum.net/topic/397835-find-slime-spawning-chunks-125/
def slimeChunk(seed, x, z):
	randseed = long(seed) + long(x * x * 0x4c1906) + long(x * 0x5ac0db) + long(z * z) * 0x4307a7L + long(z * 0x5f24f) ^ 0x3ad8025f
	r = Random(randseed)
	i = r.nextInt(10)
	return i == 0
	
def goodSeed(box, seed):
	minx = int(box.minx/16)*16
	minz = int(box.minz/16)*16

	for x in xrange(minx, box.maxx, 16):
		for z in xrange(minz, box.maxz, 16):
			if slimeChunk(seed, x/16, z/16):
				return False
				
	
	return True

inputs = (
	("Max Seed", 100000),
)

def perform(level, box, options):
	for seed in xrange(options["Max Seed"]):
		if goodSeed(box, seed):
			#level.root_tag["Data"]["RandomSeed"] = TAG_Long(seed)
			print "Found good seed: " + str(seed)
			return
	
	print "Didn't find good seed."

# Code for testing the speed of the brute-force.
class TestBox:
	minx = 0
	minz = 0
	maxx = 10*16
	maxz = 11*16
perform(False, TestBox(), {'Max Seed': inputs[0][1]})

test-results.sh

$ time python BanSlimes.original.py; time python BanSlimes.fix1.py; time python BanSlimes.fix2.py

# BanSlimes.original.py
Found good seed: 12949

real    0m1.639s
user    0m1.607s
sys     0m0.016s

# BanSlimes.fix1.py
Found good seed: 12949

real    0m0.634s
user    0m0.608s
sys     0m0.013s

# BanSlimes.fix2.py
Found good seed: 12949

real    0m0.520s
user    0m0.496s
sys     0m0.015s