Another useful one is
How Many of Me? which tells you how common a name is likely to be in the US (as of the 1990 census). I can't remember where I first heard of this site, apologies if it was here. So for Clark Kent:
There are 39,507 people in the U.S. with the first name Clark.
Statistically the 897th most popular first name.
More than 99.9 percent of people with the first name Clark are male.
There are 51,663 people in the U.S. with the last name Kent.
Statistically the 675th most popular last name.
Famous people with the last name Kent:
Jeff Kent
The guesswork part comes when it puts them together and says "There are 7 people in the U.S. named Clark Kent." - it's statistically dubious because there is no attempt to work out the ethnicity of names, or the likelihood of the combination if there are other factors such as e.g. people not wanting their son to go around with a fictional character's name.
Tried a few others
Lois Lane - 334,292 Lois, 164,107 Lane, 181 Lois Lane.
Jimmy Olsen - 293,265 Jimmy, 60,780 Olsen, 59 Jimmy Olsen
Perry White - 77,495 Perry, 847,885 White, 216 Perry White
Catherine Grant - 566,776 Catherine, 182,341 Grant, 340 Catherine Grant
Jonathan Kent - 475,606 Jonathan, 51,663 Kent, 81 Jonathan Kent
Martha Kent - 626,037 Martha, 51,663, 106 Martha Kent
Lana Lang - 44,066 Lana, 63,819 Lang, 9 Lana Lang
Lex Luthor - Neither name is common enough to do statistics.