NSArray Equivalent of Map

It only saves a couple lines, but I use a category on NSArray. You need to ensure your block never returns nil, but other than that it's a time saver for cases where -[NSArray valueForKey:] won't work.

@interface NSArray (Map)

- (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block;

@end

@implementation NSArray (Map)

- (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block {
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:[self count]];
    [self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        [result addObject:block(obj, idx)];
    }];
    return result;
}

@end

Usage is much like -[NSArray enumerateObjectsWithBlock:]:

NSArray *people = @[
                     @{ @"name": @"Bob", @"city": @"Boston" },
                     @{ @"name": @"Rob", @"city": @"Cambridge" },
                     @{ @"name": @"Robert", @"city": @"Somerville" }
                  ];
// per the original question
NSArray *names = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {
    return obj[@"name"];
}];
// (Bob, Rob, Robert)

// you can do just about anything in a block
NSArray *fancyNames = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {
    return [NSString stringWithFormat:@"%@ of %@", obj[@"name"], obj[@"city"]];
}];
// (Bob of Boston, Rob of Cambridge, Robert of Somerville)

I've no idea what that bit of Ruby does but I think you are looking for NSArray's implementation of -valueForKey:. This sends -valueForKey: to every element of the array and returns an array of the results. If the elements in the receiving array are NSDictionaries, -valueForKey: is nearly the same as -objectForKey:. It will work as long as the key doesn't start with an @

To summarize all other answers:

Ruby (as in the question):

array.map{|o| o.name}

Obj-C (with valueForKey):

[array valueForKey:@"name"];

Obj-C (with valueForKeyPath, see KVC Collection Operators):

[array valueForKeyPath:@"[collect].name"];

Obj-C (with enumerateObjectsUsingBlock):

NSMutableArray *newArray = [NSMutableArray array];
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
     [newArray addObject:[obj name]];
}];

Swift (with map, see closures)

array.map { $0.name }

And, there are a couple of libraries that allow you to handle arrays in a more functional way. CocoaPods is recommended to install other libraries.