The Roland D2 is a great way to get MC-505 type sounds and patterns, for a price much lower than what the MC-505 generally goes for. It has 157 preset patterns, 232 RPS phrases, 600 sounds, and 30 rhythm sets.
You can create patterns, then string those patterns together in song mode to complete an entire song.
I find the V-Synth style touch-surface to be a bit difficult to work with, but then again I connect my keyboard controller via midi, so I never even use the touch-surface. Here is a song using just the 8 parts on the D2:
The downsides are working with 3-character abbreviations such as Syc/rt9/Mes/t-n/ACh/Arp/rPS/SyS/thr in the display, and there is no media slot for floppy disks or SM/SD/CompactFlash cards, or anything, so you have to send/receive data via midi SysEx.
The default midi channels are as follows:
Part 1 Ch. 1
Part 2 Ch. 2
Part 3 Ch. 3
Part 4 Ch. 4
Part 5 Ch. 5
Part 6 Ch. 6
Part 7 Ch. 7
Rhythm part Ch. 10
RPS Ch. 15
Arpeggiator Ch. 16
Here are the factory patterns on the D2:
Update: It is still an impossible interface with the 3 character display, and I do not understand how that touch surface is in the least bit musical. You must have the manual so you can figure out what the settings are. But if you hook it up to a midi controller and just send patch changes via midi, then the 3 character display does not really get in the way and you get all those great sounds. This might be my recommendation for the cheapest way to get multi-channel good-quality sounds (such as synths, 808 drums, classic Roland pads) added to a keyboard controller. I think the biggest downside is that if you want to use the patterns, you have to manually transpose all the key changes to new copies of the pattern - meaning that this is not an arpeggiator, you cannot just hold down chords and have the pattern switch the key to what you are playing. It has RCA plugs instead of 1/4 jacks, but that is OK.