Various Cheats:
Bring up the console using the ~ key then put in one of the following codes:

please fly - flight mode

please ghost - No Clipping mode

please tellall - Shows Netricsa messages

please refresh - ???

please giveall - All items

please god - Invincibility

please open - All doors opened

please invisible - Invisibility

please killall - Kill enemies

Cheat method 2 (Used on some versions??)

During gameplay press F1 to get the console then put in a code below hit ENTER then press F1 again to activate

cht_bGiveAll - Get all weapons

cht_bGod - Invincibility

cht_bInvisible - Be invisible

cht_bRefresh - Max Health

cht_bFly - Fly Mode

cht_bGhost - No clipping mode

Get any level:
When in Serious Sam, hit new game. Then go to custom levels. It should list every level in the game. Simply click on one and you will be transported there. Another cool thing about this is that you have any weapons that you normally picked up on the levels you skipped.