#1
The idea: somebody posts a basic chord progression; then, people modify it to their hearts' content, and post their favorite subs.

Chord Progression 1 (just to make things relatively easy to start out with):
Cmaj7-Am7-Dm7-G7
This is a I-iv-ii-V in the key of C major.

My subs:
Cmaj7-A7-Dm7-G7
We start out with a little bit of the secondary dominant, making the A chord want to go to the Dm7 just a little bit more by making it a dominant 7 chord. Dominant chords want to move to chords whose roots are a perfect fifth below their own fifth, and D is a perfect fifth below A.

e--------------
B--8--8--6--6--
G--9--6--5--4--
D--9--7--7--5--
A-----7--5--5--
E--8-----------

Em7-A7-Dm7-G7
Cmaj7 is a tonic chord in the key of C major; the other 2 tonic chords in this key are Am7(the iv) and Em7(the iii); these 2 chords have the same function as Cmaj7 in the key of C major, though Em7 is a bit weird sometimes, but we'll ignore that fact for now. We substitute Em7 for Cmaj7. If I were to analyze this progression without knowing what the original progression was, I would analyze it as a ii-V in the key of D major followed by a ii-V in the key of C major, but we know better.

e--------------
B--------------
G--7--6--5--4--
D--5--5--3--3--
A--7-----5-----
E-----5-----3--

Em7-Eb7-Dm7-Db7
From the previous progression, we take the tritone substitutions of each of the dominant chords. This is a progression you'll see in gypsy jazz. The reason why the tritone substitution is valid is this: the 3 and b7 of a dominant chord are the notes that define the chord (the interval between them is a tritone); if we create a dominant 7 chord with its root a tritone away from the root of the original dominant 7 chord, the 3 and b7 of the new chord are the b7 and 3, respectively, of the old chord. For example, the tritone substitution of G7 (notes are: G-B-D-F) is Db7 (notes: Db-F-Ab-B).

e--------------
B--------------
G--7--6--5--4--
D--5--5--3--3--
A--7--6--5--4--
E--------------

A11-G13sus4
If we go from Em7-A7-Dm7-G7, we can consider the first 2 chords a ii-V in D major, and the second 2 chords a ii-V in C major. We can substitute a ii-V with a suspended chord built from the V chord. Though A11 isn't technically a suspended chord, if we follow the convention of not including the 3 in 11 chords, we're close enough. If that's not good enough, we can say that since we can substitute a ii-V for a V chord, we can do the opposite as well. This is actually my favorite from the ones that I've posted.

e--------------
B---3---5------
G---4---5------
D---5---3------
A--------------
E---5---3------


Let's see what the rest of you can come up with.