Domain name services come in so many forms that its hard to list them all here, but the primary ones include domain parking and catching dropping domain names.

Sedo - Review
Provides not only domain parking but acts as a domain broker as well.

Namewinner - Review
Offers an auction format to catch expiring domain names. Starting at just $15, NW can be one of the cheapest ways to acquire a dropping domain besides a manual registration.

Afternic - Review
The top domain auction site on the Internet. Also offers parking and an escrow service for deals.

Sell Domain Names - Free service that allows visitors to post their domain names for sale to help get more exposure for their domains.

SnapNames
Former kingpin in the domain backorder business. Attempt to register a domain the second it expires for about $70.

Pool.com
Replaced SnapNames as the dominant backorder services. Bids start at $60 but can snowball to thousands of dollars very quickly.

Domain Sponsor
Earn income from your domains with this top domain parking service.

DomainSpa.com
Another major parking service of note.