Recommendations for column-oriented database

Also check out Michael Stonebraker's C-store: C-store (includes links to source code and research paper)

The paper contains an excellent viewpoint on column oriented databases, that should answer most of your questions.

To quote the paper,

"Most major DBMS vendors implement record-oriented 
storage systems, where the attributes of a record (or tuple) 
are placed contiguously in storage.  With this row store 
architecture, a single disk write suffices to push all of the 
fields of a single record out to disk.  Hence, high 
performance writes are achieved, and we call a DBMS 
with a row store architecture a write-optimized system.  

In contrast, systems oriented toward ad-hoc querying 
of large amounts of data should be read-optimized.  Data 
warehouses represent one class of read-optimized system,
in which periodically a bulk load of new data is 
performed, followed by a relatively long period of ad-hoc 
queries. Other read-mostly applications include customer 
relationship management (CRM) systems, electronic 
library card catalogs, and other ad-hoc inquiry systems.  In 
such environments, a column store architecture, in which 
the values for each single column (or attribute) are stored 
contiguously, should be more efficient.  This efficiency 
has been demonstrated in the warehouse marketplace by 
products like Sybase IQ [FREN95, SYBA04], Addamark  
[ADDA04], and KDB [KDB04]. In this paper, we discuss 
the design of a column store called C-Store that includes a 
number of novel features relative to existing systems."

Infobright
It's a column oriented MySQL engine
You can use (almost) all MySQL api's/interfaces/tools but it's column oriented.

It's open-source and has a free version.
It's very good for warehousing. I had a 10Gig fact table in SQL server. Infobright compressed it to 15MB.

Sybase IQ is one I have heard of.

HBase is an open-source column-oriented database system modelled on Google's BigTable.