Just for example, I'm a big fan of the new 37signals office: http://37signals.com/svn/posts/2593-official-pictures-of-our...
I’ve worked in and designed several offices and regardless of the company size or budget, it’s important to get a few key things right:
1. Quality lighting - The more windows you can get, the better, and artificial lighting is just as important. Lighting is personal - some people like tons of light and others prefer to work in the dark. Make sure that you have some control over light variations and some adjustability in the different areas of the space (dimmers on lamps, curtains on windows, etc.)
2. A large table with plenty of seating - great for group projects, team lunches, company meetings, etc.
3. Comfortable seating - almost all companies skimp on seating. Considering a lot of people spend 10+ hours sitting down each day, $10 folding chairs are a joke. Quality seating doesn't have to be expensive, just comfy. Maybe provide each employee with $100-500 to choose his or her own chair. Great deals can be found at Ikea, Costco, or Craigslist.
4. Privacy - This doesn't necessarily mean individual offices or huge cubicles. Very few people want someone looking over their shoulder or eavesdropping on private conversations. Have enough space and spread people out so that they're not working on top of one another if at all possible.
5. Fun - It's different for every office, but fun stuff is important. A game room, a stocked fridge, a beer pong table, a TV, a gaming system, some board games, a pinball machine, a nap room, etc. Every company does it differently, but it's nice to have something that people can bond over and can provide a few minutes away from work.
6. Location - The location is important because if your employees have to commute two hours to get to your super awesome office, they’re still not going to be happy. Pick a location that provides an easy commute for as many people as possible and if the budget allows, choose a nice area. Walkability to restaurants and coffee shoppes is always good.