Poker Stars rake. This is a straightforward listing - Rake is a percentage of the pot up to a certain maximum, based on the number of players that were dealt into the hand. Right now, at .01/.02, there is a 3.5% rake with a $0.30 max per pot.
Cake Poker rake. This is a bit harder to figure out - the Rake column says how much rake is taken out, per amount in the Per Pot column that is in the pot. For instance, if rake is $0.01 and Per Pot is $0.15, there is 1 cent taken out per 15 cents in the pot. You can divide Rake by Per Pot to get the percentage. Right now, there is a 6.7% rake with a $0.50-$2.00 max, depending on the number of players sitting at the table.
I couldn't find a listing of Merge network's rake structure. Also note that a rakeback deal can lower the effective rake that you pay.
In general, below a $0.25 big blind, PokerStars seems to have the best rake structur. Higher than that, it used to be that Full Tilt + rakeback was the best. I don't know the current best for $0.25 and above.