cTwittLike (recently changed from cTwitterLike) allows you to enter any Twitter id and view the stream as they would. An interesting way to see all the tweets that they would see daily. The data is real and updated as of the time you submit their name.
Once you enter a name, you see the Twitter page with ads running down the side, quite intrusively. It runs via the Google App engine so it renders quickly as long as the Twitter API is open to the site itself. I found they were slow due to load earlier in the day