Astaro is an awesome firewall I use for about 80 users. Forget messing around with building your own on linux like everyone here is suggesting - I don't have that much free time and I'm sure your IT guy doesn't either. It's an excellent linux based appliance OS w/ a cross platform web based interface. You can either buy on astaro hardware or you can put it on an old server you own. When I was evaluating it compared really well with CheckPoint. Check it out at www.astaro.com