For me, the thing that most improves my productivity is pixels available for work. I would love to be able to connect a second external monitor to my Macbook Pro.
For now, I've settled on using my old TiBook with a external monitor, and synergy, to expand my workspace to 4 screens.
I already searched for DVI or VGA ExpressCards, but so far found none. It's a recurring query I have on Google Alerts. The best and closest thing I saw was Belkin Notebook Expansion Dock, but alas, no Mac support yet. Also, reports on the Dell site (they sell this as an add-on) are not that good.
BTW, if any of you comes across a ExpressCard/34 VGA or DVI card, please leave a comment. Thanks.