I'm going to try things like, e.g., Start Making Bingo Cards.
I actually had a few dozen buttons made in a particular design to test different calls to action against each other earlier this year, but the design itself flopped -- hugely -- against my pre-existing buttons, so much so that I didn't think color or copy changes on it would recover the loss. Curses on having my testing bandwidth limited by graphical expertise...
I'm running a test right now, with "try it for free" currently at 262 out of 792 clicking through, and "get started now" at 269/778 (so up 4.5%). It's not yet conclusive, but at no point in the test has "try it for free" outpaced "get started now".
If my results ever show themselves to be "conclusive", I'll post about them at abtests.com. I'd encourage you to try "get started now", though, to see what you find.