Secret level

A secret level is any level beyond the "fake" last level, usually levels 145-149 in a set with 149 levels (including official sets). Fake and real last levels can be changed using ChipEnd, though.

Ignore Passwords rules

The game supposedly ends after the 144th level, but Ignore Passwords can work up to level 145. The only way to reach Cake Walk is to use its password from Cypher. When level 146 or any secret level is opened, Ignore Passwords will unlock all five secret levels.

CC1 secret levels

Thanks to... (MS only)
Cake Walk (Password revealed in Cypher)
Force Field
Mind Block
Special

Thanks to... was added to the 145th slot in the MS version as a credits level, thanking the MS game designers for their work. In Lynx, Cake Walk is still level 146. Level 145 does not exist at all; it is simply a dummy that triggers the ending sequence after completing Fireflies. It can also be acessed with the password YBLI.

CCLP2 secret levels

Gauntlet (Password revealed in Cypher II)
Run-a-Muck
Cloner's Maze
Neptune
Key Color

Because the MS game engine considers all levels above 144 to be "secret" regardless of levelset, CCLP2 also has 5 "secret" levels that work the same way as those in CC1. Cypher II was made for CCLP2 to account for this, which gives the password to the first secret level, Gauntlet.

CCLP3 secret levels

Recurring Dream (Password revealed in Motion Blur)
Suspended Animation
Avalanche
Diabolical
Mr. McCallahan Presents

CCLP1 secret levels

CCLP4 secret levels

Hacked Save File (password revealed in Strandquist)
Japanese Game Show
Gimmick Isle
Gravity Well
Mental Marvel Monastery

In Custom Levelsets

In any custom level set with 149 or more levels, there are secret levels.

Trivia

The Lynx version of the game contained a Mandlebrot fractal generator. It can be considered a "secret level" of sorts, as it can only be accessed via a password (MAND).
The unfinished NES port also had a "secret level" of sorts: entering the password CRED will display two pages of credits text.