Search
Google yn Gymraeg/Google in Welsh - http://www.google.com/intl/cy/

Blogs
Agregatwr Cymraeg/Welsh Aggregator - http://blogiadur.com/

Music
Gigs + Gwybodaeth bandiau / Band info - http://curiad.org/

Social networking
Perthyn - http://www.perthyn.com/

Resources

General tips
How to set your default language in Firefox to Welsh

Add-on Welsh dictionary for Firefox: https://addons.mozilla.org/en-US/firefox/browse/type:3