On the Related Projects page of the CouchDB wiki, under "Alternatives" they mention some similar projects:
- Feather DB* CouchDB clone in java.
- StrokeDB* A CouchDB-like database written in Ruby to make embedding into Ruby apps easier.
- mongoDB A high-performance, open source, schema-free document-oriented database.
And of course Tokyo Cabinet which has already been mentioned.
There's also neo4j which is a "graph database" for java.
Of course, part of the power of CouchDB and some of the others is not just being able to store key/value pairs, but the high capacity, replication, and in particular views, which are basically the way of running queries over your documents.
If you just needed a simple key/value datastore that you can embed into your program, that doesn't have to hold gigs of data, the venerable GDBM might suit your needs.
A little hard to answer without knowing a bit more about your needs (programming language, concurrency requirements, data volumes and such).
* Web site does not appear to be working at the time of this writing.