I like the chromatic exercises, personally. I know they're not musical, but they work out all 4 left hand fingers evenly and also help develop better coordination. Of course, any 4-note-per-string patterns would work just as well, but chromatics are a good place to start since there are no difficult stretches.
It's also easy to come up with your own chromatic exercises...all you need to do is determine how your going to subdivide the beat, and set your starting tempo, and play!
Here's a nice triplet-based one that I came up with:
|-12-11-10-9---------------------------------------------------------------
|-------------13-12-11-10\9--10-11-12/13-12-11-10--------------------------
|-------------------------------------------------13-12-11-10\9--10-11-12/-
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
I usually play this exercise counting it in two different ways: (click for audio)
[a]
Triplets: 1-and-a-2-and-a ...
[b]
Sextuplets: 1-and-2-and-3-and ...
And then just carry on the pattern from there. When you run out of strings, reverse the pattern and do it ascending.
(I prefer not to skip notes when I play chromatics, so I change positions across the neck, but you don't have to)...
Another variation is to play the chromatic scale in groups of 12 notes, 4 notes per string across groups of 3 strings, in descending and then ascending order, like this:
|-12-11-10-9---------------------------------------------------------------
|------------13-12-11-10-------------13-12-11-10---------------------------
|------------------------13-12-11-10-------------13-12-11-10---------------
|------------------------------------------------------------14-13-12-11---
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|-13-12-11-10--------------------------------------------------------------
|-------------14-13-12-11-------------14-13-12-11--------------------------
|-------------------------15-14-13-12-------------15-14-13-12--------------
|-------------------------------------------------------------16-15-14-13--
And again, when you run out of strings, reverse the pattern and then just do it ascending. Audio:
Triplets and
Sextuplets
I find that by forcing these 4-note groups into 3-beat rhythms it forces you to think about which notes you accent in order to play it well.
Then there's a straight 16th note version I came up with just last night:
|-12-11-10-9---------------------------------------------------------------
|------------13-12-11-10\9--10-11-12/13-12-11-10-13-12-11-10---------------
|------------------------------------------------------------13-12-11-10---
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
|-\9-10-11-12/13-12-11-10-13-12-11-10--------------------------------------
|-------------------------------------14-13-12-11\10-11-12-13/14-13-12-11--
|--------------------------------------------------------------------------
|--------------------------------------------------------------------------
And so on.... Here's the audio for that one.
PS, I know I wrote these for 6-string....for 7-string, you can just extend the patterns one string further.