InterBase is well worth a look, we have been developing client/server apps on InterBase for about 4 years now. For the front ends we use Delphi (Windows),Kylix(Linux) or in some instances PHP. Overall we have found InterBase to be one of the most reliable SQL engines that we have come across, take a linux server with reiserfs filesystems, put Interbase on there and run an application against it pushing lots of transactions per second, now remove the power lead! ( Anybody want to try this with their sysems, just joking. You could try it on a dummy server though just to see ).
When rebooted Interbase automatically recovers with the database back on line, usually takes about a minute for a full system re-boot, how many other systems could you do this to? Makes one hell of an impressive demo.
( Perhaps that is why Interbase is used in the M1 tank, yes really. )
Version 4.0 is free on Linux ( Needs an early version, we use SuSE 6.0 )
Version 6.01 is free & open source ( Linux & Win )
For connectivity(ODBC etc) & tools, try the Phoenix web site ( The people who run this site used to be ISC )
Closing thoughts, InterBase has never had the sort of marketing that the so called commercial leaders have. Give it a try, I'm sure you'll be suprised.