Many people are reluctant to link to other web sites, for fear the visitor may not return to the original site, and for the fear that outbound links can drain Link Authority.
Neither fear is justified.
To keep from losing your visitors to an external web site, you can simply set up the outbound link to open in a new tab or a new window. Then if your visitor closes that window or tab, Bada-Bing! — they’re right back on your site where they were before they left.
As for draining your Authority, that simply doesn’t happen. And if you choose your outbound link target pages on relevance to your own content and on the quality of their content, then according to Matt Cutts (“The Google Guy”), Google’s going to reward you. [Update: Matt is no longer with Google.]
Basically, Google (as well as other search engines) wants to reward web sites that provide value to their visitors. And if your pages contain links to highly valued content that’s relevant to your visitors’ interests, Google reflects that in their opinion of your site’s value.
If you’re inclined to check out the technical discussion from Matt (and it is a bit technical) the relevant blog posting is here.