I've had The Ultimate Doom and Doom II (Xbox) since release (They came with Doom 3: Limited Collector's Edition in early April '05), and I just found out today that they have one new secret level each not found in the original PC versions.
You can find the details at ClassicDOOM.com here, and PC versions of the levels are also available for download here.
The new map for The Ultimate Doom is in Episode 1 called "Sewers," and the new Doom II map is called "Betrayal." I played through both of them today on Ultra-Violence Difficulty (Hard), and overall they were pretty good.
"Sewers" becomes the second level of The Ultimate Doom, and is a long level with many sewer tunnels to slog through filled with monsters. You end up having to fight many monsters that you normally wouldn't encounter until much later in the game, such as Cacodemons and Barons of Hell, however there's enough ammo (and a Chaingun and Rocket Launcher later in the level) to go around. Your Shotgun is your best friend here. Between the two, this one is my least favourite, as I found the level too large and generally repetative to be really great. Since it's also featured in Episode 1, which is the original Shareware episode released way back in 1993, you're also restricted to only a select group of weapons.
"Betrayal" is a very good level. It ends up being the third level of Doom II and also features many monsters you don't encounter until later in the game, such as Pain Elementals, Arachnotrons, Hell Knights, Mancubuses, and Arch-Viles. However, you also get access to the game's full arsenal, so except for a few really hard open areas (Lousy Arch-Vile), the level is challenging but not insanely difficult. "Betrayal" is better laid out than "Sewers" and features a lot of closed rooms and teleporters. The Plasma Rifle worked wonders here.
All in all, it was a nice surprise finding some minor new content for the decade plus old games. If you have the Xbox versions (also found in Doom 3: Resurrection of Evil), then go give 'em a try. If not and you have the PC versions, you can download load them from the above link.