Well, it really is a bit different. Rhythm Assembler is a drum computer script I wrote for Kontakt, remains of a cancelled project. Usually a step sequencer has a regular grid, you can use to place notes. Here, you define the grid structure, assuming there's a note at every step. It may seem backwards, but once you overcome old habits, it may be simpler to do it this way. Example, you want a four on the floor, you only need to program one step. Also, I feel, breaking down a rhythm to such formula, gives me a better understanding, of how the rhythm works.
The machine offers 8 sequence tracks, each can have a different grid sequence with different cycle loop. Grid step is defined as a multiplication or fraction of base duration, like "1 quarter note", "3 sixteenth notes", "1/3 half note" or "3/4 whole note". On top of that you can program a repeat sequence, telling the machine how many times to repeat a grid step. Considering both sequences can have a different loop cycle, it is possible to program fairly complex rhythms with a simple formula. Also the sequencer is not bound to a time signature, you can program odd, irregular rhythms, rhythms with asymmetric measures.
Another, somewhat uncommon feature of the sequencer is the "humanizer" function, which not only can delay notes by random duration, but can play them too early as well. This "timing error" function can be configured separately for each track.
And here's a quick overview video:
Rhythm Assembler is just a script, it comes with no samples and no instrument patches. There are script preset files, you can add to an existing instrument. Which is also the reason, there is no custom graphic interface and the background is black (for the workaround to disable instrument's background image). The script requires Kontakt 5.6.5 or later to work. It could be rather easily ported to any earlier version of Kontakt 5 and with some work and (loosing option to rename sounds) to Kontakt 4.
The script is free to download and use (although it's not free to re-distribute). DOWNLOAD IT HERE. To install the script, you need to locate Kontakt's user preset/scripts folder and copy the "Rhythm Assembler" directory there. There are several example presets, with pre-programmed rhythms and an "empty" preset.
There's some further beta development, you can follow here (scroll down the thread).