nanoloop & pure data. Sort of the opposites!
nanoloop (mainly 2.3 and up) just works for me, I really enjoy using it and find it fun, intuitive. I think the sequencer is brilliant, like the lo-fi FM and I always enjoy a concept (eg 4 tracks, one "poly" one noise) to conform to / break out of. (and the balance of that I guess!)
pure data is like max msp but less bull$hit. I love free open source software, especially if it can run on really cheap devices. It feels great to use such democratic tools. And besides that, It's a superb piece of software that has a wide area of use and is generally just good to know!
I use it for many, many things. Mostly for making algorithmic music and visuals with the built-in GEM library.