Note: You must have a Gameshark or a Pro Action Replay cartridge to use these codes.
I'd like to note that I'll only list the most important options of the Debug Room because there's a certain FAQ that has listed pretty much everything I'd ever want to do with this code. I'm assuming you're playing the US version of Xenogears and for that you will have to input the following code with your GameShark:
Xenogears Debug Code: 8004F34C 0000
If the code does not activate, or if you cannot deactivate the Debug Room Code from your GameShark, you'll have to input the Joker Codes, which are the following:
Now try moving around, entering another area, and soon you'll show up in the room in the pic below.
In this room, there are five doors. With the exception of the middle door (called "dokodemo" door), there's nothing too interesting in any of them. The dokodemo door will allow you to jump to almost any point in the game, including some inaccesible places or scenes that you couldn't move around in! There is nothing else to find after door #730, though (game crashes). Note that after choosing a number, you'll also have to choose a "Mappara", which is basically the spot where you appear on the scene you chose. I suggest you leave it at 0 (default), but you can take the risk of showing up inside a wall if you want.
There are also some well known faces around. Talking to Hammer to turn the background green and allow you to see every character face in the game; talking to Margie allows you to change your party, either by the regular way ("partychange") or by simply removing/adding any character you want ("memberchange"). Be sure to not press "end" before choosing your party, and note that Citan2 is Citan with the sword and Emer2 is the adult Emeralda; talking to Dan allows you to hop into your gears or leave some riding gears and some as regular characters; talking to the cow enables a music/sound test and talking to the Reaper enables you to visit other debug rooms. There are an endless number of options here, but I suggest you check at least the "Saito" room to find lots of nifty stuff.
Once again, instead of blatantly copy/paste-ing some other site that already has worked on these codes, I'll recommend CMGSCC's Xenogears Section and all its codes to alter character statuses, gears, or general items. The character and gears codes should be self-explanatory, just input the code for whatever you want. The codes for item/weapons/etc are a tad more tricky. Check, for example, the Character Accessory Modifier Codes. The first Slot will have the code 3006F4FC 00??. Now check the Accessory Digits section, and supposing you want to get a Slayer Robe, switch the ?? for 30. The Slayer Robe will show up on the first slot (the item you had there before will disappear). Repeat this process to access some rare (or impossible-to-find, in some instances) stuff!