You can write your own bot in any modern programming language using the BTC-e API. Also MtGox, Bitstamp, Bitcoin-24 and probably more popular exchanges support this.

These API's mostly give you the possibility to track the price, the market depth and place orders, making easy for you to implement your own trading bot.

Here is a starting point I found for Btc-e using Pastebin, source here http://pastebin.com/Xu64wH1e:

# BTC-e trade module takes options, pair the pair being traded eg. ltc_usd, ltc_btc,
# btc_usd. type the type of transaction buy or sell, rate the price you are buying or selling
# at with leading zero if decimal (0.025 not .025), and amount the amount to buy or sell.
#  Usage ./trade.py ltc_btc sell 0.0075 568
# Based on the already existing sample code placed on this site with my additions to make
# a working example.

import sys
import httplib
import urllib
import json
import hashlib
import hmac

# Replace these with your own API key data when generating the key be sure to check the trade
# box.
BTC_api_key = "your-api-key-here"
BTC_api_secret = "your-api-secret-here"

# Reading file to get nonce last used then increment and store for next use.
# You must create the file nonce.txt if first time using the trade api put the number 1 in it.
# It must be in the current directory as you are running the .py file in or change to correct
# path in the open(....).

with open('nonce.txt') as f:
n = int(f.read()) + 1
f.close()
with open('nonce.txt', 'w') as f:
f.write(str(n))
f.close()
nonce = n

# method name, options and nonce go into the POST parameters
params = {"method":"Trade",
      "pair" : sys.argv[1] ,
      "type" : sys.argv[2] ,
      "rate" : sys.argv[3] ,
      "amount" : sys.argv[4] ,
      "nonce": nonce}
params = urllib.urlencode(params)

# Hash the params string to produce the Sign header value
H = hmac.new(BTC_api_secret, digestmod=hashlib.sha512)
H.update(params)
sign = H.hexdigest()

headers = {"Content-type": "application/x-www-form-urlencoded",
               "Key":BTC_api_key,
               "Sign":sign}
conn = httplib.HTTPSConnection("btc-e.com")
conn.request("POST", "/tapi", params, headers)
response = conn.getresponse()

print response.status, response.reason
print json.load(response)

conn.close()