As to provide webmasters to increase their link popularity, some sites are dedicated to offer the opportunity for webmasters to swap links. Here are some of them:

LinkMarket: Over Free inclusion to their link directory. Email notification for every link exchange request. Free 10 link request per day. [*]Nearly 19,000 link partners. More features include:
Thousands of Link Partners to Choose From
No Waiting Weeks, Get Started Immediately
Fast, Easy and Free Link Exchange
Comprehensive Account Management Tools
Human Edited Link Exchange Directory
Every Reciprocal Link Exchange is Validated
International Link Exchange

Gotop: It costs $19.95 to join their link directory. Ability to send link request to all other sites at one time. About 1000 active members.

Advices on seeking link partners
Not every link counts the same. Relevant links (from sites with similar or related content) are preferred. Search engines also favor links from sites that are already popular.