Monitors are what give Sonic items during the game. There are a lot of Monitors to be pointed out, so I thought I would help you with some.

Ring Monitor
These monitors give you 10 rings.

Sonic Monitor
Sonic heads sport 1-ups.

Eggman Monitor
Look out! Only Hydrocity, Icecap, Lava Reef and Launch Base have these, but they will make you lose rings!

Shoe Monitor
This one speeds up the music, and, in turn, makes you go faster than ever.

Sparkle Monitor
Makes ya invincible for a short time.

Fire Monitor
Shrouds you in flames, making you invincible against fire attacks

Droplet Monitior
These protect you against particles, and when you're in water, you don't need air!!!

Jolt Monitor
Make rings come toward you and protect you against lightning things.