Unlocking Extras
Aside from the main storyline in the game, "Spider-Man 3" also features an additional Arena Mode, which features battles between Spidey and his fiercest foes. If you want to unlock this mode, you will first need to complete the game once. It doesn't matter which difficulty you choose, so completing the game on easy will still give you this reward.
While Spider-Man switches between his symbiote and normal suits during the course of the game, if you finish off Venom during the final mission of the game, you'll be rewarded with the black suit for use at any time in the game. No need to have to worry about filling up your rage meter.
The PlayStation 3 version of "Spider-Man 3" is the only one in which you can play through the game as the New Goblin. To do so, you first need to have the collector's edition of the game. This cost's $10 more than the standard edition, but if you prefer Harry Osborne over Peter Parker, then all you need to do is beat the New Goblin during the game's initial mission and you'll be able to play through the game as him instead of Spider-Man.
The Fight With Venom
The final battle in "Spider-Man 3" is against Venom, and beating him is not an easy task because he has the largest health bar of any previous enemy and standard punches and kicks does little to stop him. The main strategy for this fight is to stay as far away from Venom as possible when he is attacking. Don't attempt to overstay your welcome when attacking; a counter-attack can send you to an early game over. Stay airborne with your web swing ability, and when Venom has paused after an attack, get close and press "Square", "Square", "Triangle" and "Triangle". This combo will cause the most damage to him in the short amount of time you have. Repeat this retreat-and-attack strategy until you have a full Adrenaline Gauge. At this point, press "L1" and "Circle" for a powerful attack that should drain enough energy to perform the final series of moves.
Once Venom has his health low enough, you can locate metal girders around the environment. Pressing "Triangle" near these objects will cause Spider-Man to use them to stun Venom and lower his defenses. When you've done this, run over to Venom and unleash the same combo you've been using. You'll have enough time to do this multiple times before Venom is finally finished off.
Cheats
If you don't want to play through the game or any of the missions before unlocking everything the game has to offer, then visit the Cheat Menu, which is located in the Options menu and enter one of two cheats. To unlock the New Goblin for use in the game, enter "deadly". You can unlock everything in the game, including new costumes, by entering "venomsky" on the Cheat Menu.