supasnashbuhl wrote: i'd want a batterypowered one without keys cuz i already have a midikeyboard and if i ain't got powersupply i would trigger the sounds trough the mpc pads. I dont know about the electribes but i think they're not batterypowered.. Plus they got a sequencer witch i don't really need. All in all i'm kinda searching for some kind of small batterypowered sythesizer box witch can be midicontrolled.. Why can't i find anything like that? 
There aren't many battery-powered synths, but there are a few that I know of:
Chimera bc-16 - Pretty cool mini-modular patcheable synth with MIDI control.
Chimera bc-8 - Small, fixed-architecture synth.
Korg 707 - 4-octave 4-operator FM synth
Korg Poly-800 - Quirky hybrid synth, but has a powerful analog filter.
Yamaha MU-5 - General Midi module. Probably not too exciting.
Yamaha QY-10, QY-20, QY-70, etc. - XG-based synth engines.
Casio CZ-101 - Nice phase distortion synth.
Novation BassStation
Yamaha DX-100
Nintendo DS with Korg DS-10 - this would actually be a nice companion for the MPC-500
Korg Kaossillator
I would go for a Basstation or the DS-10 myself, but it all depends on what you want it for.