Utility classes.. Good or Bad?

Utility classes are ok..... as long as they don't violate design principles. Use them as happily as you'd use the core framework classes.

The classes should be well named and logical. Really they aren't so much "utility" but part of an emerging framwework that the native classes don't provide.

Using things like Extension methods can be useful as well to align functionality onto the "right" class. BUT, they can be a cause of some confusion as the extensions aren't packaged with the class they extend usually, which is not ideal, but, still, can be very useful and produce cleaner code.

You could always create an interface and use that with dependency injection with instances of classes that implement that interface instead of static classes.

The question becomes, is it really worth the effort? In some systems, the answer in yes, but in others, especially smaller ones, the answer is probably no.

From a theoretical design standpoint, I feel that Utility classes are something to be avoided when possible. They basically are no different than static classes (although slightly nicer, since they have no state).

From a practical standpoint, however, I do create these, and encourage their use when appropriate. Trying to avoid utility classes is often cumbersome, and leads to less maintainable code. However, I do try to encourage my developers to avoid these in public APIs when possible.

For example, in your case, I feel that UrlParser.ParseUrl(...) is probably better handled as a class. Look at System.Uri in the BCL - this handles a clean, easy to use interface for Uniform Resource Indentifiers, that works well, and maintains the actual state. I prefer this approach to a utility method that works on strings, and forcing the user to pass around a string, remember to validate it, etc.

I agree with some of the other responses here that it is the classic singleton which maintains a single instance of a stateful object which is to be avoided and not necessarily utility classes with no state that are evil. I also agree with Reed, that if at all possible, put these utility methods in a class where it makes sense to do so and where one would logically suspect such methods would reside. I would add, that often these static utility methods might be good candidates for extension methods.