I'd like to do monitor spanning with my Macbook Pro next to my iMac, I haven't really found a good solution, anyone use anything they can recommend? I'm using Lion. Thanks.
I personally use AirDisplay for this purpose, as it is the only application that I have found that enables the secondary screen to be able to display video or other high frame rate objects at a reasonable speed.
I have also experimented highly with Screen Recycler as well as just strait VNC which is included with the OS, however my issue with those has been with tearing of visual elements when dragging windows as well as the inability to show videos, keynotes, websites or even scroll word documents and have the second display appear fluid.
I've never tried it personally but the only software that I know of that claims to do this is "ScreenRecycler." They have a Lion version currently in beta. Here's a link.
I've used ScreenRecycler as well. More recently, AirDisplay has also been working working reasonably well for me, the advantage being interoperability between Mac, iOS, and Windows machines.
From experience in the retail environment I know that the computers would both need thunderbolt connections. I also found an article on apple's website that gives step by step intructions on how to connect two computers together and use one as a screen for another. I hope this helps.