Fanan team has released 999 - a free rhythmic MIDI arp for Windows. They say:
This little dude is actually a MIDI VSTi that creates no sound but triggers other VSTis that opened in the D.A.W. It can generate 999 seeded arpeggios which driven by **chords**. The user can set a unique speed, note's length octave range and diversity (ply) to each arpeggio. The user can also switch between arpeggios with his MIDI keyboard and randomize arpeggios in live sessions. The arpeggios are always synced to the D.A.W's BPM and time signature so the user will always get automatic positioned results.
Price: Free.