Link trading is an essential undertaking if you want your site to get a headstart in the SERPs. Its important you approach each trade keeping in mind some basic rules - target audience, anchor text, the PR of both your site and your partner site. There's several link trading tools around..

Link Vendor Value Tool -this simple tool concentrates on giving you a rough estimate of value for you links.

SEO Moz - this tool is similiar to the above tool but requires API keys, and registration, possibly a little more accurate in its results.

In terms of link trading directories:

Linkmarket.net - free link trading service

Text Link Brokers - very large datbase of link traders.

Beware of link trading services, that promise a gazillion website visitors the instant you submit your details, chances are your enrolling in some sort of link farm operation, and sooner or later the search engines will catch up with you.