What is the word for "job-hopper" with a positive connotation?

See if floater suits your needs,defined by Merriam Webster as

a worker who moves from job to job.

The positive words are for people who intend to help many companies as part of their customary job. They're called freelancers and consultants.

There is no positive word for someone who hops from employer to employer, each hiring her for a steady job and each losing her quickly. Employers hate those people, and the culture doesn't appreciate dilettantes or [goof]-offs either.

The few who would describe them positively (i.e., such people themselves) wouldn't define themselves in terms of their jobs. They'd go with free spirits, &c.

Mary is a versatile employee who has acquired a wide range of skills from short stints of employment.

You can also say that she made a lot of lateral moves as opportunities presented themselves.

versatile: 1. Capable of doing many things competently. (American Heritage)

Mary is a career butterfly who has acquired a wide range of experience from short stints of employment

might work.

Butterflies are well-liked creatures, unlike most insects, because of their beauty. They invoke pleasant memories of summer afternoons watching them flit from flower to flower. They cross-pollinate by taking pollen from one to another. This is analogous to the way in which an employee such as Mary can pick up ideas, skills and best practices from a wide range of organisations, or fields of work, and apply them to others.

On the down side a social butterfly is someone who may be seen as somewhat fickle, but there is no getting away from the fact that some people will regard job hopping negatively, whatever term is used. Butterflies are generally positive.

Here are two examples of career butterfly:

http://www.scotsman.com/news/flitting-from-job-to-job-can-give-wings-to-your-career-1-733482

http://moneyning.com/career/benefits-to-being-a-career-butterfly/

An Independent contractor provides goods or services to another entity under terms specified in a contract or within a verbal agreement. Unlike an employee, an independent contractor does not work regularly for an employer but works as and when required.

Source: Wikipedia

Transient or journeyman, but depends on situation whether that is positive.