AudioRealism Bass Line 2 (ABL2)

Introduction
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The best 303 for Mac AU with an authentic sound, with lot of presets and great usability.

GUi & Usability
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Simple and very easy to understand GUI. And you can swap to another darker skin. Most of it is very self-explanatory. 303-user will know what to do. New user to 303, will have to learn the sequencer, but it's not big thing. That's why the 303 is (among other things) so sought after. It's just very easy to get groovey sequences out of it in an instant.
The contender Phosycon from D16 has worse usability. It was confusing to me, so I wasn't able to start a sequence within one minute. But that's probably jus me ;)

Load the ABL 303 and start right away by hitting a note on your keyboard. Hit another note and listen to the next sequence.

It even supports 303-audio-file-import and recognizes and implements that pattern. I have to admit I've never tested that feature, so I can't really say how good it works.

Sound
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The sound is as you expect it from a 303. Pretty authentic, but surely not like the real deal, because a real 303 hooked up to some fine high end analog gear will still punch better.
Then there is also Phosycon, which I think has a slight better sound, but the ABL has just a better usability for me. I believe the slight sound differences won't matter in songs, that don't build upon the 303 as a main instrument (like Acid).
ABL also has two types of distortion built in to pimp the sound. They're good, but meanwhile there are better external ones.

Presets
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ABL delivers a huge amount of presets. It's so much, that I still haven't covered one percent of it. You must know, that every preset can have multiple sequences triggered by a note. There are presets for all kinds of styles. Straight bangin acid lines, funky crazy grooves, driving bass lines... you won't be bored.

Stability
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Never had any issue. Runs and runs and runs.

Cons
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There is one little issue. If you want to create a custom MIDI map, you have to reload it everytime you open the session. It is NOT saved, nor with the ABL presets, nor with the DAW presets. At least it is like this with my system: Logic 8 & OS X 10.6.8.

Conclusion
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Everytime I want a 303, I go for ABL.