Is an alternative with some features, i.e. snapshots (IIRC), then I'm planning to add custom comparators and pluggable builtin serializers. Another thing is that leveldb-ruby is built in c++, instead mine uses ruby builtin ffi, should be a little slower (benchmarks will come) but easier to maintain and to extend.

Since it uses ffi it should work on jruby, have you actually tried to see if it works well?