New mines/mortars
New mines/mortars

Replaces the mine and mortar 3D-models.
This one is more round, and has a new appearance.

New mutant tail thing

Replaces the mutant tail 3D-model.
This one is more detailed, and uses chrome mapping.

New mutant tail thing
New passenger plane

Replaces the original aeroplane at the airport.
1321 polys, improved collision shape, active wheels.

Preview | Download
New fighter plane

Replaces the fighter planes at the aircraft carrier.
884 polys, improved collision shape, active wheels.

Preview | Download
Fighter plane
Traffic Jam pack 1
Drone pack

A pack of 20 new, slightly more detailed civilian cars.
The average polycount for the cars is roughly 600.

Preview | Download
Horizon image pack

A pack of 13 horizon images for C2 tracks.

Preview | Download
Horizon pack
Menu backdrop images
New menu background images

This download contains new menu background images for the most often used menu screens.
The original images have a cyan-coloured theme, while these new background images have a darker theme.

Note 1: this pack doesn't include a replacement for the loading screen
Note 2: the font colours in menu texts won't be changed
Analog RPM Dial for C2
Analog tachometer

A replacement for the original coloured bars.
Note that the needle animation can't be very smooth, since there are only 12 different frames reserved for the animation.

Pack of new visual effects
By MuscleCarFreak - new blood puddle, bloody tire smear, oily tire smear
By Apex - new tire marks for road, dirt/grass & snow
By C2S (me) - new flames, oil puddle, powerup twinkling now blue
Preview | Download

Also, consider downloading improved explosion graphics by MuscleCarFreak, which look excellent.
(The explosions pack is hosted elsewhere, but the permission for direct linking was granted)
Edited, new powerups for C2
Customized addon powerups

Original powerups slightly edited to create new ones:

Extra ride height, Magnetic personality, Hiccups, Manual ride height control, Anti-gravity device, Blazing fast opponents, Inversed gravity device, Opponent spinner, Crazy afterburner, Mortars, Pedestrian magnet, Pedestrian spinner, Hydraulics