Jeffery Zhao published a simple benchmark of 2 'NoSQL' databases recently. In that article only basic CRU operations are compared. On macbook unibody+osx, which is the platform Jeff use, MongoDB got slightly better scores than TokyoTyrant on almost every aspect.
We're very interested in CouchDB these days, so I cloned Jeff's benchmark suite, added scripts for CouchDB, and ran the benchmark on my platform, macbook unibody+archlinux again. However the result is really interesting - it's totally the opposite - TokyoTyrant is much more faster than MongoDB on my box.
CouchDB is really slow compared to TT or MongoDB, so I just give up it after serveral round.
The only difference between Jeff's and mine platform seems operating system: he use OSX while I use linux. I'm not sure whether this is the reason we get different results, or because TT is well optimized by gcc on linux?
Try it yourself: Simple NoSQL Bench (The suite is written in Ruby)
update: After changed from Net::HTTP to Curb, couchdb benchmarks improved about 1/3. Config couchdb [uuids] algorithm to sequential (in default.ini) has no effect on result. All 3 drivers connect to database through network, but only couchdb use http protocol, this is a bottleneck, or, trade off.