Lilith

Absent anything to do for the next 38 minutes, I'm going to type a little bit about my Borderlands siren.  I spent the entirety of PT1 and most of PT2 in perpetual ammo scavenger mode, since my SMGs (especially the double anarchy) chew through whatever I can carry.  Having gone through two playthroughs and 2 DLC runs without seeing a single ammo regen mod, I instead shifted to a less bullet-intensive build the moment I got my hands on a Maliwan Hellfire.

I'm using a slightly scaled down version of this.  The basic strategy is to throw up as many DoT effects on as many enemies as possible while shooting only as much as necessary to spread the Hellfire DoT.  A corrosive artifact handles acid, the Hellfire for fire, Radiance for electricity, and Phoenix for a bonus stackable fire DoT.  With four damage effects on every enemy in the encounter, stuff dies.  Everytime something dies, with my mod, I get a 11% shield regen boost over five seconds (Girl Power).  Between that and 70% damage resistance for a few seconds coming out of phasewalk (Silent Resolve), I'm mostly immune to damage once I'm into the attack sequence.

This becomes more powerful in a few more levels since I'll be able to flesh out Blackout, which is a phasewalk cooldown reduction every time I kill something.  Once that's 5/5 I'll pretty much constantly be popping phasewalk and will always either be totally immune or effectively immune to bullets.

On top of all that, being effectively 9 ranks into Phoenix also gives me a 45% chance to not consume SMG bullets.  So ammo's no longer a problem when I'm rocking this Firefly build.

Its major weakness is any boss fight, since they can't be ignited (or have any DoT effects), and my Lilith isn't optimized for single-target non-elemental damage.  It's not a build that works for Crawmerax, and it makes me only a half-contributor to Knoxx fights (though it'd be great for the run up to Knoxx).