Can I calibrate 2 screens to match, without buying expensive gear? I use a 2010 Cinema Display and a 2011 Thunderbolt Display, both connected to my 11-inch MacBook Air. The screens are ever so slightly different in terms of how things are reproduced. 

How can I get both screens to render things the same? 

I know I could buy expensive color calibration hardware, but its really not worth it to me for a one-time use. Instead, is there any other way I can do this? I have a pretty good eye for colours (which I guess is why it bothers me in the first place?) and so perhaps there is some kind of tool that lets me do this by eyeballing it?
Personally, I'd go with the expensive option - you have a grand's-worth of screens there; why not have them both accurately profiled?
The first downside of manual calibration is not knowing which of the two is 'more correct' in the first place - the second is actually getting them both to look the same...

Back in the good old bad old CRT days I always used to set screens up by eye - since I bought a colorimeter, I haven't looked back.

If you do decide to go for a colorimeter, avoid the entry-level Huey Pro, as it no longer works on Mac. Bottom of the line these days is probably the Color Munki Smile, which is quite adequate for 'in-house' calibration, if you don't need a fully-calibrated workflow to printers/publishers etc.
You can also do PCs &amp; your iPhone with it, if you want ;-)



Anyway - if you want to try it manually, the functionality is built right into the OS itself...
System Prefs > Displays > Color > Calibrate... [you get one on each screen, though it doesn't look like you can run both at once]



Clicking that launches the Display Calibration Assistant, 



which you can run through as many times as you like; save with a different name each time &amp; review later to see which you're happiest with.

I didn't quite dare to actually run the assistant at first - didn't want to mess up my own calibration ;-) When I did try it I discovered there seem to be far fewer tweakable options than there used to be - you used to be able to adjust per 'gun' colour balance, amongst other things. All I got to pick was a white-point, which really isn't going to be of much use :/