Maybe you know I have a new hobby project I'm working on for six months now - javalibs.com, which is basically something like Google for Maven Central repository. Why I made it? Because I was sick and tired of the current state of public search engines. When creating javalibs.com I took lots of inspiration from them, but ultimately I made my own path and created some original concepts:

mvnrepository.com (a.k.a. most used)

+ search results are sorted only via Maven Central usage, which works for some libraries, but others are impossible to find (see lower for comprehensive search tests)
+ categories
+ retired artifacts are marked

- just Java dependencies from Maven Central, no plugins, archetypes, classes or other libraries from other repositories
- updated weekly

search.maven.org (a.k.a. official)

+ updated every hour or so
+ search for classes

- terrible search engine (see lower for comprehensive search tests)
- no plugins or archetypes

javalibs.com (a.k.a. new contender)

+ pretty good search engine
+ autocomplete
+ retired artifacts are marked
+ dependencies, plugins, archetypes, java classes
+ libraries from Maven Central, Spring, JBoss and PrimeFaces Maven repository

- updated weekly

libraries.io (a.k.a. search everything, find nothing)

+ much more than just Java libraries - javascript, ruby etc.
+ very nice integration with GitHub

- from Java libraries just Maven Central
- terrible search engine (see lower for comprehensive search tests)
- it seems there's no update mechanism right now, often it doesn't contain current information

bintray.com (a.k.a. Maven Central alternative)

+ has a little bit more than just Java libraries from Central (jcenter)

- seldom updated
- terrible search engine (see lower for comprehensive search tests)

maven-repository.com (a.k.a. mostly unknown)

+ dependencies, plugins, archetypes
+ updated daily

- terrible search engine (see lower for comprehensive search tests)

Others, mostly unknown

• http://findartifact.rubeko.com/ - very basic with minimalistic design and functionality
• http://findmaven.net - seldom updated, terrible design, terrible search engine

Search engines search capabilities:

There are total 7 tests which tested the search engine to it's limits. Note that on search.maven.org I didn't use "Advanced search", because it requires precise knowledge what you're looking for, it's extra hassle and most people don't know about that feature anyways. If the result I was looking for was within several first results (first 5 results if not specified otherwise), the search engine would receive one point, otherwise zero. These are summary results:

Tests: