What's the female equivalent for the informal ぼく? I've heard that females will tend to use わたし, but then it's said to be formal. Doesn't there exist a female informal "I"?
Yes, there are several.
This is just a casual form of わたし used very commonly by young women. However, it has a more "adult" feel to it then うち, as I feel women between 20 to 40 yrs old use it more.
This would be second on the list of most common, especially with young teenagers. I wouldn't expect a women past 30 yrs+ to use this form. It originated in Kansai-dialect, but has spread around among teenagers. There is a phenomenon were dialectical expressions become incorporated into slang of teenagers.
This is much rarer for women, but I've heard it used by women who are into sports and that are more masculine.
This one originated from the Tohoku area in Honshu. It is more dialectical, but there are women who use it.
The most common of them all. It can be used in many situations.
This is more common in children, but some women use it to sound cute (although some people despise it).
A mother or grandmother might also refer to themselves as おかあさん ・おばあさん also when talking to there children.
You should keep in mind that each of these forms give off different nuances. For example, some people feel うち and あたし are used by "unintelligent people", and shouldn't be used. Also, I might missed some that are used in regional dialects that I'm unaware of. Wikipedia has a huge list. Japanese pronouns are very complex!
わたし is used by women and girls in both formal and informal contexts; it is not strictly reserved for formal or polite speech. In addition to わたし, younger women often use the variant あたし in informal situations.
Another informal personal pronoun that women use is