Beat you to it
Cysill is the Welsh spelling and grammar checker
Cysgeir is the Welsh/English set of dictionaries
Both are Windows applications and trial versions can be download from
http://www.bangor.ac.uk/ar/cb/
However, in answer to the original question, a Welsh language web search page, try this one -
http://www.gwgl.co.uk
Before anyone asks, yes, I can speak Welsh but as this thread was started in English I'll reply in English