you can download nmap (from repos) and run:
sudo nmap -P0 -O www.someaddress.com
Doesn't always give a clear reading, but you can mostly make out the OS.
kim@Haze:~$ sudo nmap -P0 -O www.google.com
Starting Nmap 4.20 ( http://insecure.org ) at 2008-01-20 14:41 CET
Warning: Hostname www.google.com resolves to 3 IPs. Using 184.108.40.206.
Interesting ports on nf-in-f104.google.com (220.127.116.11):
Not shown: 1693 filtered ports
PORT STATE SERVICE
Device type: general purpose
Running (JUST GUESSING) : Linux 1.X (86%)
Aggressive OS guesses: Linux 1.3.20 (x86) (86%)
No exact OS matches for host (test conditions non-ideal).
OS detection performed. Please report any incorrect results at http://insecure.org/nmap/submit/ .
Nmap finished: 1 IP address (1 host up) scanned in 35.718 seconds