If you ever wanted to be able to step sequence any parameter in Live, have a look at StepOne. If it looks familiar, that's because it is based on the Arp One by Max for Cats as well as some functions of the LFO device.
You might be familiar with Nico Starke's "ML-185 Stage Controlled Sequencer". Now there's an updated version of it available for free. The most notable change in the MK16-185 Sequencer are the 16 steps instead of 8, an often requested feature.
Genstep16 is a very interesting kind of step sequencer. It has the ability to generate new arpeggio/melody patterns over time. Also adding variations in velocity and duration make the patterns seem very organic and human. Very inspiring!
Polyrhythmus is a Max for Live rhythm sequencer, arpeggio builder as well as a MIDI CC and parameter automator. If you're into polyrhythms or arpeggios, this one's quite a treat. It looks great, too and despite all this is free.
I personally prefer real analogue synths, but they're often not exactly cheap. If you're looking for a modular synth that integrates seamlessly in Live and you have Max for Live, Dope Matrix may just be the thing for you. It's a combination of a sampled Live Pack with a Max for LiveStep Sequencer for Push, APC40 & Launchpad.