How to lock screen besides pressing Nexus S right side power button
There are many widgets available on the Android Market that provide this functionality.  

If you are running a custom ROM on your Nexus S (e.g. CyanogenMod) you might already have this feature within the Notification power widget.  Go to Menu -&gt; Settings -&gt; CyanogenMod Settings -&gt; Interface -&gt; Notification power widget -&gt; Widget buttons and enable the Go to sleep button (also enable the widget itself in the previous screen.)  Now when you pull down the notification bar you can tap the "zzZ" button to lock the phone.