Synergy won't precisely give you what you've specifically asked for - but perhaps in your particular case it'll actually fit the bill nicely.
Synergy allows you to share a single keyboard & mouse from one machine with many. You set up zones where as your mouse hits the edge of one screen, it ends up controlling another machine. It's really quite awesome. It's cross platform (Windows, OSX, Linux) and is fairly easy to setup (and is free).
EDIT: Here's some crayon between the dots regarding how Synergy could ever 'fit the bill nicely'. Jad didn't explain his use case, so we don't know how he intends on using the second monitor. If the extra monitor is to add pixels for a single application, then my Synergy suggestion quite obviously won't help. However, suppose he's programming something and wants a screen just for reference documentation.. Then Synergy is extremely helpful (this is what I've used Synergy for during the past ~5 years). Hence, as I stated on the first line, while this doesn't explicitly resolve the question Jad has asked, it may well resolve the actual need he has that's inspired this question.
There's lots of videos on youtube regarding Synergy you can check out: http://www.youtube.com/results?search_query=synergy
So with Synergy (vs what you actually asked for) you'd be missing the ability to drag windows/applications between machines. Obviously, you can set up regular file sharing to move documents/content between machines. Synergy supports basic clipboard integration (you can 'copy' a line of text on one machine, then 'paste' it on another). Anyway, perhaps that'll meet your actual need that inspired the question. Maybe not - could end up helping someone else who reads the question wondering about their options along these lines - hence my posting this answer.