Being 'freelance' I'm always looking for places that have free wifi so I can escape the dreaded cabin fever of my flat. Prague was my 'office' for a few months last year and I would recommend it to anyone. It has great coffee which they stop serving at around 5 or 6 when it turns into a lovely spot for cosy red wines and chats.
The only thing I would say is watch out for the food - the platter consists of some bread like stuff, some platted cheese and pickled pate - I don't know if this is how they do things in Prague for real but I don't know that it translates so well.