One of the things we discussed in this month's writing group was names. The person being critiqued had chosen some names that started with the same letter and had the same number of syllables. We cautioned her to keep the characters far apart in the narrative, lest she confuse readers.
I've had a similar problem in my own writing. I have a character whose name I haven't figured out yet. I've figured out which names don't work, because I've been typing up scenes where she interacts with other characters, and I've gotten confused about who's doing what. That's a really bad sign. If you must have characters whose names begin with a similar sound, at least make them have different numbers of syllables.
The best name sources, though, are baby name sites and census information. If I want a character to have a common name, I look at the top 25 most common names for the year the character was born. If I want the character's name to give a hint to his or her identity, I do a reverse lookup on a baby name site for the definition I want, and look for a name that fits. I keep a running tally of character's names, especially ones who interact, and I make sure they're not going to be confused with one another.
With last names, I go for rhythm. A character with a one-syllable first and last name is flat and uninteresting, which fits for the characters who are trying to have a low profile. A character who's a handful might get a complicated first and last name. Generally, though, I try to balance them. A polysyllabic first name will usually get matched up with a one-syllable last name, and vice versa. Also, I never start a last name with a vowel if the person's first name ends with one, and I make sure names don't otherwise blend. As someone who's been called "Allison" for most of my life, I'm rather emphatic on that point. Do not use names that blend together, unless you want people mishearing your character's name within the narrative.
I did stumble across one more resource for character names: the Everchanging Book of Names. It's a free download, and they encourage comments about its utility, so I would urge you to do so if you use it. It's of particular use for those of you writing fantasy or science fiction.
Most baby books and name census information will be available with a quick Google search, and there isn't one in particular I use. This one is useful in that it allows you to specify the number of syllables and what the name begins and ends with. This site allows you to search by meaning. Almost all of the sites want to congratulate you on your pregnancy, so be prepared to ignore everything but the name generator portion of the page. (From the comments, there's also Behind the Name, which allows you to search by nationality as well as meaning.)