In the northern parts of China 250 (二百五) is used to refer to someone as not so smart. This probably stems from the fact that in order to say 250 in Chinese, you don’t say er bai wu, you say liang bai wu, but Wikipedia says different, and who am I to argue.
[Apparently I'm wrong again after chatting to some of my friends, they say wiki is right, and to call someone 500 (五百) is even worse - you're twice as 二百五！ There was talk about that when 伍佰 (the Taiwanese Singer) was big here.]

For example 56 (五六) sounds like 无聊 (bored) in standard mandarin, and I lost a conversation about the origins of the video site 56.com name to someone on Twitter.
55 also has a meaning which roughly corresponds to an affirmative exclamation of excitement eg yeah! woohoo!

38 (三八) is used as a derogatory term for women, and typically means bitch.
This stems from March 8th (3月8日) being womens day (三八节).

三八线 on the other hand is the 38th Parallel, which is the dividing line between North and South Korea for those that didn’t know that.

In school however, the 三八线 was the dividing line on the desk (although this did have its origins in the North / South Korean divider line.

There is a an amazingly well done video cartoon below which recalls the 三八线 to illustrate primary school life. Unfortunately no subtitles, but its eminently watchable without.

More video’s here over at Kuanger http://www.kuanger.com/, or do a Tudou search for 哐哐哐

Lastly, but not least is 88 – this sounds like bye bye, so is often used in online chat when you sign out.

I hope you enjoyed the lesson, and 88!