LinkedIn Free for basic membership
LinkedIn is an excellent professional networking site.
You can upload/download your contact lists from/to Outlook. There is an excellent LinkedIn toolbar for Outlook which integrates Outlook with your LinkedIn account. You get a free public web-profile page. Your contacts can post endorsements (testimonials) on items in your profile. There is a job-search function, and a people search function. There is an interesting Question & Answer forum. There are special interest groups. If you want to contact people with whom you have no direct connection, you can do so via your own contacts.
Xing Free for basic membership
Xing is a professional networking site, based in Germany.
It supports multiple languages, although much of the traffic is in German.
Facebook Free for basic membership
Facebook is a networking site oriented mainly for students, and for social networking.
It has excellent facilities for uploading photos into galleries, but be aware that the Terms and Conditions state that Facebook owns the copyright of any photos that you upload.
Be prepared to have a lot of your time wasted with games, quizzes and general spam.