BOOMERANG was the first instrument to measure fine details in the CMB. WMAP was able to do the same thing, except using data covering the whole sky:
It turns out that the power in the CMB at various angular scales (the spatial Fourier Transform of the CMB map, more or less), is distributed into so-called 'acoustic peaks'. Here's a plot of the acoustic power spectrum of the CMB:
If you've seen a colloquium or talk on cosmology in the last 5 years or so, you've probably seen this data.
Anyway, the red fitted line in the plot is the prediction for a universe in which the total energy budget is approximately 70 percent cosmological constant, 25 percent cold dark matter, and about 5 percent normal 'baryonic' matter (e.g. protons and neutrons). This plot can also be used to find the total energy density by measuring where the first, big peak occurs. You can see in the plot that the peak is at about 1 degree of angular separation, and looking at the data by eye you can see that most of the structures are separated by distances of about a degree. A 1 degree angular separation corresponds to a universe which has exactly the 'critical' energy density and exhibits no spatial curvature. The measurement errors are small, less than 1 percent.