I haven't really found a particularly good alternative. I used to use AnySoftKeyboard, but stopped after I found that it was the cause of some performance issues I was having that would occasionally cause characters to be entered out-of-order on a slower Android device.
The lack of decent open source onscreen keyboards was actually one of my larger surprises when I hit Android. I thought that there'd be keyboards with user-specified macro keys, keys that could be dragged into a new layout or added and removed, etc, but no.