I've just been using LinkedIn companion for Firefox for the past few days. It's been very useful so far, especially integrating with my webmail accounts to show me LinkedIn profiles of the people who email me.

• See LinkedIn profiles for anyone sending you Web email
• Search your LinkedIn network from any web site
• Share articles you find on the web with your LinkedIn network
• Bookmark profiles and searches for easy access
• Find a job by seeing who in your network can connect you to hiring managers while you browse major on line job websites

You can find the extension here.

What other software or websites have you found useful for networking? Let us know in the comments!