I'm personally using a Western Digital MyBook (500 GB) and I have had no problems. In terms of brand, I'd recommend Western Digital or Maxtor. Both are very reputable companies and make good products. Price wise, I think I paid $135 for mine and they go on sale all the time. Granted, mine is the basic model. There are higher end models which do the exact same thing but come with backup software. However, unless you need the software, I wouldn't sped the extra money. It's not worth it.
Now that the recommendations are done, I'd like to throw in one very important caution. Never, ever move your laptop with the hard drive connected to it, especially if the drive is spinning. I made the mistake of thinking I had more cable than I did and dropped my hard drive four or so feet to the ground while it was being written to. Unfortunately, this caused a head crash (physical damage to the disk) and I had to replace it. While they are fairly sturdy, shock like that is a sure-fire way to destroy your drive so just be careful when moving around.
Anyway, I hope this helps. It's been a long day and I'm pretty tired so if I forgot something or this post ends up being somewhat incoherent, I apologize. Just let me know if you need anything else.
